Is lower RPM always more efficient?
 

One benefit to having a larger displacement engine is more power, if you tune it right you can get more low end torque which can mean lower revs but this leads to more fuel usage. A low displacement engine is great for better fuel economy because it allows for less fuel to be burnt and has less mass to move around but that leads to less power. One way to compromise is to add a turbo, you can get some of the fuel economy of the low displacement engine while having the power of the higher displacement engine. One article I read off of hotrod.com said it simply that having a turbo basically increases the displacement of the engine (not literally, but effectively since you're going about atmospheric pressure). This made me realize that a turbo is bad for when you want the benefits of low displacement as you'll have more fuel coming than you want but it also creates parasitic drag on the exhaust.

Alright, so here is my question:
In this SPECIFIC situation and no other, is it better for a low displacement engine vehicle at a fixed speed when meeting a grade to have to downshift, raising the RPMS with no turbo at all (anywhere) OR is it better to have a low speed turbo that allows the low displacement engine to maintain its speed despite the grade? For example instead of having to downshift and run the engine at 2500-4000rpm while climbing the grade, you could instead maintain your RPM of 1000-1500? My thinking is that being able to maintain the RPM of 1000-1500 with the usage of a turbo would be better due to not needing to move the mass in the engine so quickly and would save more overall despite the now extra fuel entering into the combustion chamber.

On another forum, a driver with a Saab turbo swears that his fuel economy is terrible if he stays in a high gear and opens the throttle more, which apparently puts him in boost, and that his fuel economy is much better if he downshifts to run at higher RPM and stay out of turbo.

I'd guess that a modern turbo car with the computer-controlled turbo is more efficient if you do it like him, downshift enough to stay out of boost.

In my slightly torquey NA car, I've only found ONE situation where downshifting is more economical: Climbing a hill at WOT at 1200RPM and not accelerating, if I downshift one gear and climb at 1450RPM with less throttle, I get a good 40% fuel economy increase. Apart from that I have not found a situation where my car gets better fuel economy in a lower gear than necessary to keep the engine from growling.

There is another thing to take into consideration when you are talking about throttle position and economy on top of what you mentioned.

The ECU. Almost all engine computers go into an open-loop power enrichment mode. I believe the normal throttle position for that is 80% or more. You also lose active timing advance and that's why the majority of cars take off faster from a stop at part throttle. Economy will drop substantially in that enrichment mode.

Take the Durango for instance, it makes plenty of torque down low and has no problem with hills but I can see better mileage by climbing a hill in 3rd gear revved to 3200 as opposed to at the edge of an overdrive downshift at 1900 rpm(at ~70mph).

I am going to try that and watch my scangauge more closely.

I usually just get my speed up so I don't need to downshift and stay at 70+ going up and just blow by the trucks in the truck lanes....haha ....Then on the way down I just DFCO in 3rd to keep from using the brakes or I would be off the road.

Yeah I've noticed an improvement in fuel economy from driving a little faster up a grade in order to maintain the tallest gear than maintaining that slower speed and having the car downshift. In my FJ cruiser, when doing 5-7% grades, at 55mph it will tend to be quick to downshift as soon as possible, but those same grades at even 60mph I can maintain my gear. Also I've found that even really steep like 15% grades, if I accelerate to 70-75mph, I can maintain my gear better than at 55mph and I believe I've seen better fuel economy as a result but I can't tell cause I don't have a computer to check.

In the Volvo S60 I drive, on the 23% grade I have to drive up in order to get home, while the peak torque is 1500rpm, according to the computer, the way to get the best mileage is to accelerate up to 40mph before getting up the hill and then flooring it all the way up, slowly losing speed and hoping that you don't lose too much speed before you get to the top (24mph is the point where it'll downshift out of 3rd gear w/torque converter lockup).

I would think there's a point at which the velocity of the air charge in the runners and ports would get too turbulent and make a motor lower its fuel economy...cruising in my old Focus, it got its best economy right at about 1500rpm. At 1200 it was about 10mpg less...

My Dad had a '73 Mach 1 with a 351 Cobra Jet motor. The damn thing had ports so big you could stick your fist in 'em, and under about 2k the air flow was so slow through those ports that the engine just didn't run (might have with port FI - fuel wasn't staying suspended in the air). It's for this reason, I theorize, that the 5-speed he put in didn't bring his FE up at all, at 60mph he was running 1700rpm and got the same 13mpg he got before.

Actually one factor I totally forgot to realize was BSFC (Brake Specific Fuel Consumption). If you have poor BSFC numbers at a specific RPM, then at high load and in need of a lot of power, in order to get X amount of power at 1500rpm you'll need a lot more fuel than at a higher RPM where you so happen to have a good/better BSFC.. Also like another poster mentioned, if you don't have to experience the extra fuel enrichment from full throttle which you may need when you have a poor BSFC, then you could get better numbers.

I guess the solution would be to adjust the cam timing and other related factors in order to make peak BSFC at the desired RPM and not at the higher end where you don't necessarily want it. It's really possible that while the turbo was active at that poster's RPM, being at that RPM had a poorer BSFC score than at the higher RPM.

BSFC isn't a factor; it's a measurement of everything else this discussion is about. If you have a 3d BSFC map of your engine then you don't need anything else we've discussed.

Is there anything wrong with this map? https://ecomodder.com/imgs/geo-1L-bsfc-chart-orig.gif

Or is that a 3D map plotted onto a 2D plane vs. the line chart with X and Y axis..?

That chart is hard to see. It has RPM and torque, but does it have fuel rate?

The more I think about it, the more work it becomes trying to use a BSFC map to figure it out...but the data would be there. I wonder if it would be possible to feed a known 3d BSFC map into a program that's hooked up to OBDII along with some environmental sensors for detecting grade....well, that's way too complicated.

The rings are labeled with the specific fuel consumption. Hard to see is right!

maximilian, any thoughts on feeding a BSFC map into a computer hooked up to OBDII? It sounds like your kind of project. :)

I found a good map for a VW 1.9L TDI to use as an example of a map we'd all be able to use. It's got peak efficiency plotted, which totally makes everything easy:

https://pics.tdiclub.com/data/4034/TD...ith-torque.png

"Granny" Marvin and he gets 20mpg, give him between 2K and 3K RPM resulting in fairly "brisk" takeoffs and he does 25mpg... but there's an auto tranny involved there.

Since this thing is real long (haven't heard that since my honeymoon), the answer to your question is,

YES... ALMOST ALWAYS. But without a good FE readout, you'll never know.

I don't follow your logic there. More power doesn't mean lower gas mileage. If you have more power, and you use it, then you can expect lower FE. Otherwise, they are only obtusely unrelated. To say that tuning an engine for more lower-end power causes lower FE is the same logic that dictates "trailer parks cause tornadoes." The proof is all around us, the only problem being that the conclusion is not supported by the evidence.

I like the thinking, but think more 3-dimensionally. If you add a turbo, then you must lower the engine's C/R (compression ratio.) So before you even get started, you're throwing away FE with this approach because you're throwing away efficiency. However, I think there is a way this type engine can be made to work. More on that in a minute. First...

SIDEBAR:
On a gasoline engine, any type of supercharging (mechanical or turbo) adds power but lowers efficiency. As a double-aside, on a Diesel, adding a turbocharger actually increases efficiency. Now, back to gasoline.

They way I envision a small turbo engine working is by leaving the C/R as high as is practical, and fighting combustion anomalies (pre-ignition &/or detonation), not with higher octane fuel, but rather, with (insert Alfred Hitchcock-like pregnant pause here...)

ADI

ADI = Anti-Detonation Injection... aka, water injection. (actually a water/methanol mixture... essentially windshield washer fluid. check labels, some are perfect right outta the jug) Here's how it works now, followed by How I Wish It Worked.

THE STATUS QUO:
At present, most cars use a peizo-electric sensor to tell the engine's ECU that it's just picked up on a "ping" event somewhere among the cylinders. So, the ECU pulls timing advance out 'til the "ping" stops, then checks back at a later date to see how things are going, and whether its okay to return the closed-loop timing map. This could be a spitfire-rapid "check-and-adjust--check-and-adjust-etc.." sequence, or some (I believe) just ramp the timing way back and hold it there for a while. Then, FINALLY, it feeds timing back in. I am told by ScanGauge owners that the second stoopid way is how my Prius does it. I think there's a better way, and man-oh-man, will it ever save some fuel!!!

WATER INTO WHINE:
Anywhoo, the way I see ADI working most effectively is my SmartCar (for instance) would run, instead of its current gas-sucking Big-Block 1-liter engine, a turbocharged 500cc engine. And that engine would have approximately a 14:1 C/R, which would be fine at low-throttle conditions. So, with this new power paradigm, when I need some giddyap and press the pedal while using my normal cheap-*ss regular gas, the sensor notifies the same ECU of the same "ping" event," only this time, instead of pulling the timing back (which it reserves the right to do), it first tells the variable-rpm DC motor on my homemade ADI kit open the ADI solenoid and let the meth/water cool mixture things off a bit. The pressure (and therefore, the flow) would be tailored to whatever the throttle position is at the time of the "ping," and could increase/decrease very quickly to get the most out of the meth/water mixture injected. If this doesn't work (as when some idiot forgets to top-off the ADI tank, which Could/Would/Should be the already-in-place windshield washer tank), the ECU would, as a last ditch attempt to keep things under control, pull some timing out. It could even cut the throttle postion since almost everything is now going throttle-by-wire.

LET THEM EAT METHANOL:
In the above ADI scenario, we can have most of our cake (high C/R) and retain control of our silverware too (regular gas). During low throttle applications (example: 45-mph flatland cruise) such an engine would require no ADI when using a 14:1 (or more) C/R. In fact, you could run your car right now on 15:1 if you'll promise to never press the throttle more than 15%. But when you really needed the power, the ADI would step in and keep a lid on combustion chamber temps. Win/Win.

And just to keep you from thinkin' I must be some kinda bloomin' genius (you WERE thinking that, weren't you?), my daddy was the best GM mechanic in the southeastern USA from the mid-1950s 'til 1966, when he left the auto world in order to get well-paid for keeping industrial machines making money. Well, in 1962 (I think it was... I was in the 2nd grade), he drove home a turbocharged aluminum V8 Oldsmobile F-85 with a strange "bag" on the firewall. This bag contained a special mixture which, conveniently, you could get from your Oldsmobile parts department. Daddy told me how a turbocharger worked and what the ADI did, and how it did it. So, while I've kicked the idea around in my head for 47 years now, I'm a bit late to the rodeo. Thing is, I've experimented with it over the years, but never knew how to source the hardware that would allow super-vaporization of the ADI mixture, or even if such hardware existed. Well it's a new day, and all the new EFI fuel-injector technology has changed all that, hasn't it?

Not really a factor.

Imagine that you have among your tach & speedo, a "power meter." (don't laugh... they exist and I invented one 25-years ago while sitting in Mr. Moses Bernard Jr.'s Algebra class. I also drew up a working prototype of what is now Audi's Dual Shaft Gearbox. Still waitin' on my check...). Someone is now making a version of my very idea... but they didn't get the idea from me so I'm not getting my monthly check!). So, let's assume a 200 horsepower engine, a manual trans (keep it simple) and a steady 5% grade (you'd be surprised at how steep that is).

Now, to hold a steady 60mph during a 5th gear climb, let's assume that we already know that it's going to require that your 200-hp engine produce 90-horsepower. The way to get it is to watch the power meter and press the throttle until the meter reads 90-hp. Result? Steady 60-mph climb with no wasted fuel. But there are choices, meaning you can:

1) stay in 5th and keep pushing the throttle down 'til the meter reads "90-hp" or,

2) downshift to 4th (increasing revs) and press the pedal a bit less than the above scenario would require.

So, which gives the best mileage?

THE ANSWER:
Staying in 5th will return better mileage... every time. Lower pumping losses is the reason, and it is irrefutable. Before you fire up the flamethrower with what you might consider to be "conflicting data," consider this:

If you've ever tried this and wish to refute my above statement, then you did it without the "power meter," and that means that you--more than likely--saw the hill, felt the strain, and pressed 'til your (non-existent) power meter read 105-horsepower. IOW, you gave it too much pedal and the effect of the extra hp you used was "dulled" by the climb. This means you were not really holding steady, but rather, accelerating. But on a steep climb you might think that moving from 60-mph to 62-mph over a 5-mile stretch might not qualify as "acceleration," but believe me, your engine sure sees it as such! Sometimes it's such a close thing that there is NO discernible speed difference between 1/2 throttle and full-throttle, but there sure is a FE difference!!! And if you get into that situation, as has been stated elsewhere, your ECU starts throwing up the white flag of "Richen the mixture, boys!!! The cap'n wants FULL THROTTLE... AGAIN!!!"

Long story short, what I do is try the hill in 5th. If it takes more than about 1/2 throttle to make the climb, I'll move to the right lane (when available) and drop the Smart back to 4th gear... taking the hit in FE. Why? Because high throttle settings causes high combustion pressure which, in turn, presses the rings out harder against the cylinder walls... not to mention causing higher EGT & moving you closer to detonation. You're not going to kill a car on one big climb, but I have a weakness for taking care of my machinery. Besides, if you take time to look at my Gas Log you'll see I've done pretty well in the mountains, probably because when dropping back to 4th, I decrease (Interstate) speed from 70-mph to around 50-mph or so. The lower speed offsets some of the fuel lost by using 4th gear, and my toy car is happier.

SIDEBAR #2:
Don't have a power meter? Perhaps you do: A fuel flow meter (intantaneous fuel mileage readout... like my Prius has) is, essentially, the same thing.

Didn't mean for this to be so long, so...

So Long,

ADiF

PS: I'm told by some friends in-the-know that,
1) Don't use tap water. It'll work but has too many oil-contaminating minerals, and,

2) Don't bother with a homemade kit. Okay, so they didn't tell me the last one, but back in 1979 I did this to a 2.0-liter Datsun 510 engine that had the WORST combustion chamber design since the Harley-Davidson Sportster. No power 'til you REALLY cranked the timing up, at which point it sounded like a rock crusher being fed chunks of cast iron. I used a salvage GM windshield washer pump and made it adjustable with Mikuni (motorcycle) carburetor jets. It worked, but not optimally. Unless you REALLY have some pressure/vaporization going on, it takes a heckuvva lot of water to make much difference. The kits now available have the goods (Snow is one respected maker), they're just expensive... probably because the reverse-baseball-hat crowd buy them to keep their 1200-hp B-16 Honda Civics from doing a Chernobyl after 3-seconds of full-throttle. Also, with a "real" unit, I'd be willing to bet the minerals in tap water would really ream the intestines of those teeny-tiny precision orifices in the injector.

something else to consider: (a rant about my truck...) engine speed vs FE vs vehicle speed... my truck seems to get better mileage the faster i go with more weight in it on the highway. a few weeks ago I did a 150 mile round trip at about 65-70 mph @ 3/4 throttle most of the first half because i had 1300lbs (plus me) in my little truck. (coming back was empty). got the best tank since last fall (and no it wasn't unusually low on either side).

few things that might contribute to that: extra weight + good wheel bearings and solid axle makes for VERY long DFCO coasts downhill. it's throttle body injected so higher throttle=better air/fuel mix. tiny injector. low output, low rpm engine so at those rpm it's around peak airflow for the head or getting close. very undersquare cylinders: low piston speed for the rpm. comes together so to rev it a little higher and use a little gas gets more miles for that amount of gas yadi-yadi-ya so better mpg.

I'm sure sitting 2" lower and having 'stuff' stacked up a little higher than bed height helped aero too BUT this is something that keeps happening when I load it up and go fast even if it's under the tonneau

Do you get better mileage only when you load it up and go fast or do you get better mileage when you go fast? Are you carefully checking your mileage? For all we know, your truck has better BSFC is at a slightly higher rpm and load.

Assuming a healthy engine with proper mixture control over itself, when considering the hows-&-whys of mileage, aero drag (A/D hereafter) trumps all else. As for fuel mileage; speed kills... every time. If there is a situation that seems to run counter to this, then there is some LARGE variable being overlooked.

Rolling resistance (R/R) is next, followed by weight. These are inescapable constants. R/R's increase with speed is linear; with more tires on the road there is more R/R... a lot more.

Weight hurts steady mileage less, but for every downhill where weight (balanced against A/D) gives you a X amount of "shove," you pay 1.5X (guessing here) to get to the top of that hill in order to collect it.

I'm not doubting your gas mileage increase under the conditions you stated. But again, assuming a healthy engine with good mixture/timing control, there's something you are overlooking.

I'd guess driving technique.

I thought I was a pretty good throttle jockey when it came to fuel mileage... and in a way, I was. In Toyota's 1975 (or '74; it was during Toyota's "See How Much Car Your Money Can Buy" ad campaign) Gas Mileage Rodeo, every dealer had a Corolla with a glass jar hanging out the passenger side window. After filling out a form, a salesman accompanied you on a short trip to see how far you could get on the quart of gas. When you were ready to go, the salesman would twist an aluminum knob at the bottle's bottom, and gasoline was fed into the fuel line from only the bottle. The winner at at each dealership went on to represent that store in the state finals, held, in my case, at Darlington motor speedway.

IMPORTANT RECONNAISANCE NOTE:
Previous to signing in, I had asked what the record was, then drove the course to see WHERE that record run would have ended. Then I drove.

I set a new record, but it was only VERY slightly farther than 2nd place. In my earlier scoping of the course, I noticed there was a slight uphill just after record run's finish. What I was trying to do was make it OVER that hill, which would add, not a few hundred yards, but more like 1/2 mile. For whoever could get over The Hill at 35-or-so MPH, a near-quarter mile downhill coasting session awaited. I made it partially up that cursed slope, then ran dry. So, I told my buddy Keith (at the time, we were partners in a German car garage) how close I'd come and, given the chance, what I'd do to make it over The Hill.

Armed with knowledge and my tactical plan, Keith signed up and went into battle. The tactics (and his driving skill) put him over the top, and he set a record distance that was not to be equaled for the rest of the rodeo. In fact, no one else even came within a half-mile of Keith's record! Most never made it to the foot of the grade but, for those who did, The Hill had stopped 'em all dead in the road.

I said all that, not to invite (ahem!) glory, but rather, to illustrate that I had at least SOME hypermiling chops, even then. But here's where the story is really going...

Until I got my Prius (with full-time, real-time mileage info) I had no idea how sloppy was my technique, and how much mileage I was giving away. For the first three weeks, on my standard 50-mile loop, I couldn't get above about 58-mpg average. When I finally figured out how to read and "use" the real-time AND cumulative data I had, I got to where the Prius's 100-mpg FE readout frustrated me as I could drive for hours at a time with the gauge topped out, with no idea if I were getting "just" 100-mpg, or 115. All was technique, made possible by (in my case), the real time data... which--I'm assuming--you don't have on your truck.

And there's where the story went. With real-time data, I nearly doubled my fuel mileage. Part of the increase came from slowing down, but I can now drive the same loop AT THE SAME SPEED THAT ONCE GAVE ME 58-mpg, and average nearly 80.

It's not your truck "liking" high speed. It's not the aero efficiency gained or lost by the truck's ground clearance. It's not the efficiency of throttle-body EFI at higher openings. It's all your driving technique, which cannot be accounted for without some real data.

IOW, you do your best mileage with the truck/trailer/route/speed combo stated. Then turn the keys over to me (or anyone else, including yourself) with real time data available, and any of us--you included--could return at least 25-30% better fuel mileage. If your wife gave absolutely ZERO feedback when you hugged her, you'd assume you were doing something wrong, and you'd find something she DID like. With no feedback, a relationship would be a difficult (and boring) thing. Data! That's the thing you need.

To sum up: I'm pretty sure it's driving technique that's making your numbers go counter-intuitive to the known laws governing such things. In the words of Pogo (he of old-time cartoon strip fame), "I have seen the enemy, and he is us!"

ADiF

I've read a few posts from people providing well-researched data about aerodynamic flow, where there are two different types of flow. There is a speed on some cars where the flow type changes. I recall one post about a Corvette that gained a significant increase in FE at or above 70mph. In the discussion I think it was proposed that most cars do not get the same effect until a much higher, less attainable speed.

Of course, now I can't find them, so all that isn't even theory, merely a vague memory.

R/R's increase is linear with speed, but then so is distance's increase. Aerodynamic drag is definitely the speed-sensitive factor.

How could you go at the EXACT same speed and get 80mpg instead of 58mpg? You said instant feedback is important, well how would you know you're getting 58mpg unless you had some form of instant feedback? Are you now using cruise control when before you didn't? I found the best gains (for me at least in my automatic) is in using cruise control.

Same (or close) AVERAGE speed for my test loop. And the answer to the question is this: Hill Technique. This is where all my mileage was going, and why stoopid, wasteful (but handy!) cruise control can beat most peoples gas mileage numbers... and up 'til recently that included my own.

That's what the whole story was about. The Prius DOES have this feedback, and his truck (most likely) does not.

Fortunate that you should mention that. In my Prius (with real-time fuel mileage feedback), hilly country always killed me. Oh, I got okay mileage, but always thought someone could do much better, but with all my trying, it wasn't me. Then, on a 1800-mile round trip, I let my COMPLETELY non-gearhead buddy drive. After attaining road speed in hilly Pennsylvania country, first thing he asked was, "Where's the cruise control?" Being very used to this type of territory (I live 40-miles from the Smoky mountains), I shuddered at the horrible mileage that was about to show itself, but let Lee drive as he wished. After the first half-hour, my jaw dropped.

His mileage chart (the Prius makes--and holds--a 30-minute spreadsheet-like display of mileage stats) was better than what I could've gotten using my "magically-calibrated" gas-mileage foot. So I begin to think of the best way to get the Prius up a hill economically, and all the things that I must surely have been missing. Now, after using the Prius's cruise control (which I NEVER used to do, in ANY car), and watching how it works, I've arrived at the point where I can now beat the cruise control's FE numbers... rather handily, I might add.

The point of this story is not how clever I am, but rather, it seems to me that no matter what I think I know, inspiration and learning continue to come from wherever they come from, keeping me plenty humble about my gas mileage numbers and the skill-level applied thereto.

If you can't beat cruise control numbers, then your driving technique is sloppy and ill-informed. And before you bristle-up at this insinuation, recall that I--a mileage guru in my area for over FORTY YEARS-- have been constantly shown new blindspots in my skillset. And from all indications, I expect it to continue.

An extraordinarily good machinist friend of mine (he and one other guy did ALL the head-machining work for ALL Jack Roush race engines... or was it Robert Yates? ...whoever Dale Jarret drove for when he was almost unbeatable, Chuck did the heads) used to tell me, "Ray, if it takes too much time to CHECK your work, then just assume it's perfect." Of course, his tongue was in his cheek, but the idea is, for you to check your work, YOU MUST HAVE REAL-TIME FEEDBACK. As good as I once thought I was, before the Prius, I was sloppy and ill-informed. I'm now STAYING around 60+mpg in my Smart Car, and I owe it all to the driving lessons I received from:

1) Reading this (and other) lists

2) Doing my best to listen (not "instruct") folks like Wayne Gerdes and, instead of entering a peeing contest (imagine two dogs walking slowly in a circle, sniffing each other's behinds trying to figure out which one of 'em's about to die), I read, corresponded, and listened.

3) My Prius's on-board real-time data acquisition and display. It's why I bought the car! Everyone knows that the only thing REALLY WRONG with a Prius is the insufferable pr*cks who buy them! (and I'm really hoping to exclude myself on this one!) The car has been an education to me, one who has been modifying cars and motorcycles (and even other things with engines!) for utmost efficiency, cleanest combustion, and best use of resources expended since 1964.

BTW: I was only nine-y/o then, but my daddy (whom everyone he ever worked with called, "The best d*mn mechanic I've ever seen. It's like he can talk to the engine/transmission/carburetor(s) and they tell him what to do!") was there to explain carburetors, timing curves, you name it.

He never hung around working "with" me, but no matter how tired and dirty he was after work (coal miners were cleaner!) he made time to explain something to me so that I could understand how to fix it without his help.

You older chaps will remember there was no A/C in those days (in the cars OR in the shops), and transmission rebuilds, babbit rod overhauls, and valve jobs were a DAILY thing, and ONE MECHANIC had to do everything. If you couldn't run an AAMCO cylinder boring cabinet, turn a crankshaft, knurl pistons, grind valves & seats, align a front end and not just "get it in spec," but fix the "pull to the left" or the odd tire wear pattern, and turn brake drums to be REALLY round, you didn't get work at a major brand dealership except somewhere down near the apprentice level. And you'd better progress pretty quickly or the other mechanics would get tired of "carrying" you.

No, he wasn't Superman. Your town used to be stocked with people just like him. They're almost all gone now. And how-the-heck did I get this far off the thread?

With good feedback, I can whoop the pants off any cruise control I've ever seen... so far. But only last year, this was NOT the case.

Knowledge: go grab it by the tail and don't let it go 'til they start shovelin' dirt in your face. Only then is it time to stop and rest.

ADiF

I'm just hypothesizing how and why.

I doubt any one of those factors would help alone and assume it's the combination. No I don't have real-time data, it's an '87 so real-time readouts are a little more involved. Just trends: 3 or 4 tanks of hard use highway that are 1 mpg better than slower steadier highway and a few tanks from hauling heavy loads like that. Which brings me to driving styles like you mentioned: driving hard and fast at 70-80 gets better mpg (tank) than steady 60, 65, 70, etc. loaded to the tits at the same speed doesn't seem to bother it much.

Kind of like a friend of mine whos car, by all rights, shouldn't be running for the 180k miles, missing/broken wires, vacuum leaks, lack of tuning, and random parts thrown at it. Yet somehow, with an automatic trans to boot, it dyno'd better hp and torque at the wheels than the original advertised @flywheel power specs.

Driving style is pretty similar empty or loaded (except the mentioned hard/fast driving with friends late at night in the city when everyone thinks the freeway's part of the autobahn). the biggest difference is that when loaded down like the latest heavy trip the throttle can be open over 50% just maintaining a steady 65 mph. Accordingly it'll vary from 0-100% depending on hills and traffic wheras empty it'll be 25% to cruise (guess) and vary 0-75% with WOT not really happening. Reaction to hills about the same either way... steady throttle unless speed gets really low.

My mention of the bearings is that it's got tapered rollers in front, recently repacked with amsoil grease and readjusted by yours truly which made a noticeable difference coasting and a solid axle in back which means straight rollers. straight rollers are VERY efficient and won't increase friction nearly as much with weight as any other style bearing. Tires are kept at about 35 psi (sidewall max), I think I upped it to 40 for the latest heavy load.

BUT my truck aside, all other things being equal lower rpm will yield better mpg until you get them so low the engine can't make enough power to cruise. (Like F-bodies and other cars using the T-56 6-speed... you can't even use 6th till you're going at least 70 mph. but once you get there it's doing 1700 rpm @ 80 mph :-p)

Does this logic translate to lean burn? Substitute going out of lean burn with turbo.... I've been wondering how much better fuel economy the VX gets in lean burn at a given rpm compared to WOT at same RPM.