To get more FE...drive faster?
 

Hello all, I've been lurking but this is my first post.



Today I got my best tank ever-31 mpg

It's my highest. Second highest was 30 mpg. I've been keeping track of tank mileage for quite some time now. Here's the kicker. The 30 mpg tank was on a trip back from my brother's place, the 31 mpg tank just occurred on the way back to Findlay Ohio from Detroit. Both tanks happened at a rough average speed of 75-80 mph. You shoulda seen me slingshotting around semis...

Obviously this flies in the face of conventional wisdom that says you must drive slow to get good mpg. In fact, I've been driving like a grandmother lately for just that reason, and I think my best tank in the last 2 months has been 26 or so. Tonight I just wanted to get home yesterday, so imagine my surprise.

The car may have something to do with it. It's a 95 Buick Skylark 4 door 4 cylinder automatic.

Has anybody ever heard of this happening? I've spent the last 3 months staying at or below the limit on the highway, and now I feel silly.

You should get a ScanGauge if you don't already have one. There are many variables when driving. With a ScanGauge you could easily test steady state MPG at varying speeds... or see short term averages for P&G techniques.

My Accord's steady state fuel economy is much better over 60 than under 50 MPH. I think it would do crazy P&Gs at a low average speed.

Anyway, your situation could have involved a 15 mph tailwind... maybe it was just more steady state, maybe it cleaned some carbon? It's tough to tell.

Does it have an ODBII port? It should...

no OBD2 started in 96-97 should be a OBD1 car

that might be becuase those were all highway that might be why your getting such high fe. Do you do all highway on those 26mpg trips?

it could be that the engine is fully warmed, or that your automatic transmition is locking up at the higher speeds, and not alwas at lower speeds, or it could be the time spent behind semis, on long trips your tires warm up rasing the presure, your engine warms up fully, along with the fluid in your tranny, so it simply could be the long trip.

Yeah, there's LOTS of variables that could have made that difference. Next thing on my FE todo list is to select a good test course and run it at different average speeds (60-65-70 or whatever) just to get some science on the thing. I kinda wonder if the higher RPM/ speed gets the slushbox working more efficiently.

I'd LOVE a friggen ScanGuage, but I'm definitely OBD1. Exactly how do I get myself a SuperMID?

Both in top gear? I find that difficult to believe.

My T-bird's FE is best at 45 (Around 45-50 mpg), it gets worse slower, and it gets worse higher, until you hit around 63, then the FE jumps up to 40-45, then starts dropping off again as you go higher still.

I don't know a lot about turbo motors, but I wouldn't be surprised to see a different shape curve on a speed vs. mpg graph in top gear, compared to a naturally aspirated motor like the Accord's.

I suspect a turbo motor's efficiency changes much more dramatically with varying load levels.

Speed isn't the reason behind this, it's engine RPM. Your engine is far more efficient at whatever RPM you are at 80MPH than at 60MPH. Engines have a best efficiency RPM, and running in it can more than counter wind resistance, to a point. Ideally, you'd want to regear your car so that you hit that engine's sweet spot at a lower speed, which would allow the engine to run at its most efficient without the wind resistance.

Two examples, had an 89 Camaro RS 305, with an ADS superchip in it, no other changes. Running 93 octane, it would give me 30MPG at 80MPH on cross country driving. If I drove slower, mileage dropped off, if I drove faster, mileage dropped off considerably. Second example, had a 79 GMC halfton, with a mildly hopped up 350. At 60MPH it pulled down a steady 14MPG. I changed the TH350 3-speed auto trans out for a 700R4 4 speed overdrive, and mileage dropped at 60MPH to about 12MPG. If I ran 75 or so, it got 14MPG again. The truck had 2.92 rear gears, and with the 700 the engine lugged a little at 60MPH. Had I switched from the 2.92s to 3.23s, I'd most likely have been seeing 17MPG at 60MPH since the truck would have been back in its optimal powerband at 60MPH instead of 80MPH, and I'd not have been fighting nearly as much wind to boot.

Hope this helps.

Most gas engines likely create more hp per unit of fuel as top gear (highway) speeds increase and the engine is run closer to its BSFC peak, but that's not quite the same as saying "your engine will use less fuel at 80 mph than at 60 mph".

Modded V8's / drivetrains are going to have very different BSFC maps.

I still find it hard to believe that a 5-speed 4-cyl Accord uses less fuel at 60+ mph than 50, steady state.

Quite Possible
 

Hi Folks,

Although I'm new here, I do believe that what the original poster is saying is quite possible. Let's look at a simple scenario - How do we get 30 MPG:

1) Travel 30 MPH at a flow rate of 1 GPH
2) Travel 60 MPH at a flow rate or 2 GPH
3) Travel 80 MPH at a flow rate of 2.67 GPH

So, from case 2 to case 3 the fuel flow increases 33%, but he's gotten there 33% faster, because he's going 33% faster. That's the simple case. Now let's complicate it a bit.

When you increase the velocity, the drag increases as a square of that. However, maybe the engine and tranny also runs more efficiently at the higher RPM, so that it counteracts the increased wind resistance. The end result may be that the increased efficiency overcomes the increased drag (i.e. the flow rate doesn't increase linearly with the speed of the vehicle).

It's not that he's using less fuel at 60+ mph. It's that he's getting more distance out of the increased amount of fuel he's using, which changes the MPG ratio.

How can you scientifically test this?

Start at point A, drive to point B at an average speed of 75 MPH, then drive back. Measure the fuel economy. Then start at point A, drive to point B at 55 MPG, then drive back. Do both trials under the same conditions. Post your results.

You can also use a ScanGuage to watch your fuel efficiency, but that's already been said, and IIRC you have OBD1, not OBD2, so that won't work for you.

Cheers,

Joe

I think the main thing to consider is the trip...
My best tank ever was on a full highway trip drafting a bus...

If you do the same exact trip slower than you could compare your mpg. But comparing my highway trip where there is no stop and go traffic, not having to warm up the engine etc, to my daily commute is no comparison.

You didn't say anything about the automatic tanny: Does it have a lock up torque converter? If it doesn't, there could be a lot of torque converter slip at lower speeds, and this would torpedo your mileage at low speeds.

A similar thing happens in my F350. The torque converter will simply not lock up under about 45 mph, no matter how much I baby the throttle.

I've been looking for a chip to reprogram the tranny, but no luck so far.

It can, but this tends not to happen with a couple exceptions. Big vehicles with big engines and auto trannys, since getting the tcc to lock up can really increase efficiency, and may not happen at lower loads. And... relatively aerodynamic cars with really big engines and manual transmissions, since the change in BSFC compared to load can match or outpace the change in energy compared to speed. However, just about every other car/engine combo out there can and probably does have a drop in efficiency wrt over most of it's OD/top gear range.

My CRX is getting the same FE at an average of 75mph on the fwy as it did when I was averaging 65mph. This is averaged out on multiple fillups, so I can only conclude that the HF engine is more efficient at 2500 rpm then it is at 2000 rpm to make up for the increased drag.

It's not related to engine speed much, if at all. It's related to engine load. Provided your statement is in fact correct, since I'm not there to verify it and I have no way of knowing what you are and aren't controlling for. ;)

Edit- I'ma callin' BS. A stock 49 or CA HX should have near ideal gearing. Imo there's no way it'll get exactly the same FE at an average of 75mph compared to 65mph provided a controlled test.

It's not inconceivable at all. My Dodge van with 360 got its best FE at 70mph. I could never break 15.5 @ 55, 60, or 65mph, but at 70mph I was pulling nearly 17mpg.

I think it has alot to do with rpms as the cam timing and intake manifold resonances are rpm related. At 2500 rpm, it may be running more efficiently because of this.

Could be... But not according to every singe BSFC map I've ever seen. I've never seen an engine geared properly that will increase efficiency by 30% if it's already near or greater than half load and minimal BSFC. Course, it could be that Honda designed a weird engine unlike any other engine, but if I had to pick between that and a lack of consistent testing on your behalf, I'm sure you can guess which one I'd pick. ;)

https://metrompg.com/posts/photos/gcc-autobild1.gif

I think it's time to invoke Sagan:

Anyone claiming their common, modern car gets better fuel economy the faster they go is an exception to the rule. Extraordinary claims require extraordinary proof. Plot it, as above.

It just doesn't make sense in the case of the Skylark, Accord or CRX mentioned so far.

My Accord's steady state fuel economy is much better over 60 than under 50 MPH.


Yes, 5th gear.

I found it hard to believe as well... I would have sworn 45mph/70kph would have been the sweet spot for the Accord. This was with rolling terrain mostly using CC going up past Barrie and Huntsville from the Toronto area on the long weekend. It was most likely a combination of 4 people in the car and the rolling hills. The engine seemed to be below it's sweet spot when driving at what would seem to be great FE speeds.

Best speed seemed to be 55mph, but going anywhere from 60 to 65 didn't seem to reduce mileage much. In comparison, dropping down to 50 really increased consumption on any upward pitches. I wish that ScanGauge had an altimeter and could graph stored data like my Polar hear rate monitor / bike computer!!! ;-)

If I ever take the Accord on the 401 west of London where the road is pretty well dead flat forever I'll report back the CC sweet spot.

I'm not sure why you are implying I have a lack of consistent testing. I only drive my CRX on my commute and drive my Integra on the weekends so for 99% of my driving, the car is driven on the exact same route, every day, same amount of hwy/city (I'm guessing about 70/30) driving. In my gas log below, arrow A is where I started using the same gas pump at the same station. From this point on, you can see that my mileage, though not wonderful for a CRX HF, is pretty consistent. From Arrow A to Arrow B I would try to shift below 2000rpm and keep the fwy speed below 65. From arrow B on, I'm shifting between 2500 and 3000rpm and the freeway speed between 75 and 80. Before I changed the speed shift points there's 1430 miles/32.88 gallons for an average of 43.46mpg and after I raised the speed there's 3588 miles/81.09 gallons for an average of 43.97mpg.

I have not changed anything in the car.

This is real world driving, averaged over sevaral hundred miles, unless you put the car on a test track with alot of instrumentation, I don't see how I'm going to get alot more consistent. To me, the numbers don't lie, I'm getting about the same mileage after increasing the speed.

https://www.fuelly.com/attachments/fo...63a9dfdd4d.gif

Ah! CC. That changes everything. Particularly with the hills & added weight.

If you'd been using your foot & brain (if traffic had permitted), you would have greatly exceeded the CC's fuel consumption in those circumstances while maintaining the same average speed.

I can guarantee that on level roads at a constant speed your Accord gets better top gear fuel economy at 50 mph vs 60+, and better still at 40.

You need a SG or SM, a deserted section of road, a bunch fo runs, and consistent weather to do an accurate comparison. Traffic alone can screw with a test in a ton of ways, not to mention any changes in weather, fill ups, etc... If you're trying to compare tanks while commuting it's nearly unpossible to do so in an accurate matter. :thumbup: Especially if you're a typical CA driver... :p

I would would agree with you on one or two gas tanks, but I think I have enough miles at each speed to average out the weather/fillups/traffic etc to show if the mileage were worse at the higher speeds I would see it.

Not unless you can control for average speed, time idling, acceleration, energy lost via braking, etc... accurately. Ime on busy CA freeways, traffic will clump in groups and tunnel, except for commercial stuff. There is no hope of getting a controlled comparison from that mess, ever. Imo of course... ;)

(A) What added weight? We actually had an empty seat! You sound single... LOL

(B) My legs are cramped in the Accord - CC is my survival tool, so I'm very much guilty of trying to keep my feet out of it.

My brain however was very much FE engaged, much to the chagrin of my wife who was very relieved after the long weekend when "she" removed my ScanGauge from her car!

I had fun trying to find the sweet spot for CC in mildly rolling terrain. I was testing raw cruise and also assisted cruise (tap speed back on climbs). Tapping back had minimal impact when the approach speeds were near 50 or lower - unless it was a very small rise. Getting back to speed after seemed thirstier until it got closer to its sweet spot.

(C) I totally agree, but terrain and traffic are facts of life in most driving scenarios. Like I said, if I get the chance to ever test it on flatter terrain, I'd like to know what the optimum steady state speed is on flat stuff. Too bad that it won't be the Accord going to Florida in December... I can't believe I said that!!! ;-)

(A) Guilty!

(B) You've got to do what you've got to do. Glad to see you're at least playing with DWL somewhat through the CC controls. I've done that a bit, but my experience is cruise can be brutally abrupt, and hard to tame that way.

Anyhoo, a waaay different picture has emerged with the added context you've given that was missing from the simple statement "My Accord's steady state fuel economy is much better over 60 than under 50 MPH."

were you driving in a pack of cars? If you are going 75-80mph with a bunch of other cars, and you're all driving close to one another--that is, a long tailgating train of drafting, you will get much better FE than if you are driving 75-80 without any cars around. You mentioned slingshotting from 18wheelers - obviously drafting behind these is good FE.

A 4cylinder engine, even on a buick, should not be more efficient at speeds above 65mph. my guess the "culprit" is tail wind and/or pack driving.

I have a relatively small aerodynamic car with a long overdrive. At 70mph its turning just under 2500rpms. Peak torque is 2400rpms. I have found that the slower you go the better mileage you get. In rolling country hills two lane roads I can average about 60mpg. I limit minimum speed to 35mph up hill.

I can get around 50mpg at 60-65mph. I can get 60mpg at 42mph. Last tank was 62mpg scangauge reported 29mph average. Thats the nature of the beast.

I find that on a busy freeway I can ease in with a large group of semis doing a 2.5-3 second draft and get 50 mpg at 68-72mph. I was also able to drive out of KS city last weekend for 200 miles at 70+mph and held 49mpg for over 200 miles no drafting. It was a tailwind.

Conclusion, although driving slow puts the engine well outside its most effiecent range I still net better FE.

PurpleHaze, you'll just have to test it some more.
If 75 mph really does get you great FE in that car you'll see it happen regularly if you keep your speed up.

I suspect it's otherwise - but you'll see how it goes.
Any time I run my car faster than usual I see the FE drop the next time I fill up.
ymmv, as always.

Assuming no changes to your car, I suspect a good tailwind may have helped you going Detroit to Findlay, Ohio. I think you were going mostly East, and the prevailing winds are west to east so it's quite possible. A 15 mph tailwind would give you the air resistance at 75 that you'd nornally get at 60. Which is a huge change really.

Keep tracking those numbers.
Did you start a gaslog here?

Top speed is relevant but so is how you attain that speed. Driving 'like a grandmother' you accelerate slower and spend longer doing so. Driving 'spiritedly' you accelerate quicker and spend less time doing so.
It's possible to use less fuel by getting to speed quicker even if you put the foot down to do so. (Sorry if that's obvious to a lot of people but it's worth throwing in!)

Folks, shoulda-woulda-coulda comments are all well and good, but real world trumps theory every time. In theory a bumblebee can't fly because its wings are too small, but it buzzes from flower to flower anyway. Luckily bees are not engineers, otherwise they'd starve to death. The man says he saw better fuel economy at a higher speed. I believe it's possible based on my own driving, and feel that he has something to pursue here. One tank is not that accurate an indicator, but if he can duplicate the same results over 3 tanks, or preferrably 5 tanks, then it's a fact.

My own results with speed vs economy were found over months of driving, not one shining tank. The Camaro that gave me 30MPG at 80MPH was over the course of 6 months of driving from Maryland to South Carolina and back every weekend, all interstate. My beloved, betrayed 79 GMC's results were an average over 5 years and 3 different transmissions.

Fact is, all engines have their sweet spot, and two otherwise identical engines will have different efficiencies for reasons that I don't know. In theory, two identically built engines should have the same power and fuel economy ratios, but in reality one will blow the other one off the road and net 5MPG better doing it and a third identical engine is even worse. You will generally get the best fuel economy when running the engine at its preferred RPM in high gear regardless of speed, to the point that wind resistance just pushes it down. The only way to find this spot is to experiment, once you find it you can make hard part changes to adjust the speed/RPM down to where you are running that RPM at the speed you run the most. If your car gets its best mileage at 80MPH, regearing the car so you hit that RPM at 60MPH will show a mileage increase because you are then running the car at its most efficient RPM, with less wind to push.

Best way I can see to actually determine the engine's best efficiency RPM would be to strap it down to a dyno, then measure fuel usage at different speeds. You'd need to load the car down to simulate road resistance, but that would be steady. Once you find that efficient RPM, gear the car to run at that RPM at your most used speed, might see a 10MPG jump off that alone on a small car, provided it isn't already geared close to that.

What theory are you talking about? :confused:
No one here is stating anything about theory AFAIK. We're just relating to other posters based on our experience and known confounds. W/o consistent runs w/ little to no variation it's hard to pick up what does what, however if we isolate what we're trying to track down and test it, then maybe we can figure out what does what provided it's within our ability to detect it. Saying that speed hurts efficiency ain't no theory, this is documented observation. Every single vehicle I've ever driven, or seen tested, has a drop in mpg compared to speed in top gear at highway speeds. If you don't think yours does, test it, in a controlled environment, and post the documented results up. I'm sure we'll all be interested in it, since I for one haven't seen any vehicles documented that increase mileage with speed at highway speeds.
https://www.metrompg.com/posts/photos/gcc-autobild1.gif

Sorry 'bout the hotlink Metro. :p

"Saying that speed hurts efficiency ain't no theory, this is documented observation."

I think everyone here is talking about optimal spots in the MPG envelope... if you extend all those lines on the graph below 50mph, at some point they'll drop as well.

In "real world" driving conditions around where I live with the cruise on, I was very surprised to find lower FE at 80kph (50mph) than at 100kph (62mph), 90/55 was best, and I agree totally with you on the theory aspect. The power to overcome aerodynamic drag is exponential. That's a fact nobody can dispute.

No prob - I already linked that image up at post #21. :)

I don't think they will - as long as the car stays in top gear above engine lugging RPM. But the context for that to be true is: measuring FE under constant load (e.g. steady speed, level road).

https://www.metrompg.com/posts/photos...ed-chart-z.gif

As I stated in my earlier post, I have consistent runs with little variation over thousands of miles and am getting about the same mpg at an average of 75mph as I was at 65mph. This is about as stable results as we can get in real world driving. Your response was I was not getting consistent runs due to differences in traffic/weather etc. If this is the case then we should be disputing nearly every mpg claim on this site. You choose not to believe my results and personally, I could care less. I know what I am seeing at the gas pump. On the same token, I could tape up my front grill, claim a 10% increase in mpg, and because the general belief here that that grill blocks increase FE, I'd be amazed if anyone here would make similar claims that my results were caused by inconsistent testing.

In general, it cannot be disputed that at higher speeds the FE will decrease, but there can be exceptions to the rule and not every car on the road may have smooth curves as in the ones shown in the plot Metrompg posted. The curve also could be a little misleading since other then the Prius (a hybrid) all other cars on that chart are turbocharged or supercharged. A normally aspirated, non variable valve timing engine will run more efficiently at the rpm determined by the intake and cam timing. Depending on how aerodynamic the car is and the differences in the torque curves, it is possible that at small differences in speed, a car's mpg could increase or remain the same. I'm sure if I averaged 85 or 95, my mpg would fall of dramatically, or at 45 or 55, it probably would improve, but from 65-75, I see no difference.

The difference is, our mpg claims aren't comparisons. They're just claims. We all deal with different traffic and road conditions in different vehicles. What you're claiming is that going 65mph compared to 75mph doesn't produce a difference in mileage, which it may not for you because of your driving habits, conditions, vehicle, etc... But it does in every single controlled test I've seen. It's tantamount to someone who drives nothing but highway saying that killing the engine at idle doesn't increase mileage. Sure, it may not increase mileage for them because they seldom idle, but it does compared to idling, and if a driver idles enough, they'll see an increase.

If you really want to test it, I have some great two lane low traffic roads up here we can do bidirectional runs on. All you need is yerself, yer car, and a SM. Until then, because every mileage/speed test I've ever seen, and every BSFC map compared to glider/gearing indicates that mileage will drop as highway speed in top gear increases, I don't believe your assertion that your car shows no drop in mileage from 75mpg to 65mph. It may show no drop for you due to your driving conditions, but unless you test it w/o confounds, no way meng, Iyam a skeptic.