Better highway gas mileage at high speed
 

I'm baffled. Plain and simple. Here are the numbers and then I'll go from there.


Averaging 60 mph highway with some playing around in 30 miles total of twisties and approximately 8 miles total of driving less than 30 mph on a dirt road. There was about a 15 mph head wind however.

317 miles total
11.855 gallons used
26.73 mpg


Averaging 70-75 mph for 160 miles (I was tired and wanted to get back home ASAP) with about a 10 mph side wind, and then for 168 miles just my daily commute. My daily commute consists of 22 miles each way in rush hour traffic in the Dallas/Fort Worth metroplex. I go through 24 stop lights in each direction, my top speed is usually 60 mph on 2 seperate 5 mile stretches of road. In the mornings, traffic isn't too bad, but in the afternoons, sometimes I sit at some of the stop lights for 2-3 cycles before I finally get through.

328 miles
11.876 gallons used
27.62 mpg


Now, in my old 1991 Cavalier, it would get 42 mpg at 80 mph, but only get 35 mpg at 60 mph! However, it was a 4 cylinder 5 speed manual coupe. My current 1991 Cavalier is a 3.2L V6 with a 5 speed manual and it is a station wagon. It also has a turbocharger and a nice Crower cam.

At 50 mph the car is at 2000 rpm in 5th gear, at 65 mph it is at 2500 rpm in 5th, and at 3000 rpm the car is at 80 mph with a redline on the motor of 6000 rpm.

I wonder if it's possible that the turbocharger and cam together help to improve the car's VE as the rpms climb therefore increasing mpg. Cruising at 60 mph, I was seeing an average of 10-12 inches of vacuum. Cruising at 75 mph, I saw an average of 14-16 inches of vacuum.

Does anyone have an explanation for this?

Sometimes the increase in engine efficiency by running the engine a little harder and faster to get the higher speed outweighs the increased drag on the car.

My ex's 1996 Cavalier would get about 35mpg at 60mph but around 39mpg at 75-80. It had the same OHV 2.2 your old 1991 cav probably did but had over 200k miles. To my suprise it used almost zero oil between changes. The most I ever saw it use was half a quart in 5000 all city miles.

Hate to bring it up again but the Corvette is similar. The engine is so huge and grossly inefficient at making the 10hp needed to keep it at 60mph(still not bad at 28-32mpg for what it is) that taking it up to 100mph and needing 20-30hp doesn't drop your mileage by much at all because of the increased efficiency. Vette owners report that long trips in the triple digits still get above 25mpg if the speed doesn't vary too much(it's still a V8). They also say mileage usually gradually increases til about 85 where it levels off and starts to fall again around 110.

It all depends on the car, I believe every car has an optimum speed and this speed will vary from one to the next... But that someone is getting better mpg at 80 vs. 60 I find that a bit harsh, perhaps there exist other reasons such as:

- Is the car 'lugging' in 5th at 60?
> If so it might do better at 64 mph, it isn't so much a matter of a higher speed giving better mpg, it is a matter we went from 60 straight to 80...
Heck why not just compare 15 mph to 90?
I would be for testing various speeds, in between too...

It takes time, but it helps to develop a feel for the car.
Each car is different, but they all have a 'feel' to them.
Once you get this feel, once you nail it down for a car it becomes a whole another twist of a story concerning improved FE.

Things such as 'driving with load' can help spot that feel.

isnt it obvious? the faster you travel the further you go. higher gear keeps same rpm as lower gear @ lower speed. so your using roughly same amt. fuel but going alot further. thus huge mpg. right? maybe i missed sumthin though:confused:

ya missed the other posts that explained each car has its peak efficiency speed for mpg. My car is about 68-70mph, depending on grades.

Sometimes aero features don't "work" below a certain speed.

He is still in top gear at 55mph or 80mph, so that point is invalid.

Another thing to consider is the BSFC/Load curve. As the load on the engine increases, pumping losses decrease (wider throttle opening), and BSFC falls. The "sweet" spot on any car is a function of how quickly drag increases and how much BSFC falls at higher loads. I stands to reason that a low-drag car would have a that "sweet spot" at a higher speed. Or a car with an engine having a relatively flat BSFC/Load curve would have the "sweet spot" at a lower speed.

8307c4, yes my 4 cylinder Cavalier would lug quit often while traveling at 60 mph in 5th gear. My Wagonstein though, it doesn't know what the word lug means! I can put the car into 5th gear at 30 mph and smoothly, almost efortlessly accelerate up to whatever speed I need to be at.


I did notice that the throttle position was more open while traveling at higher speeds, but the vacuum level in the intake manifold was higher.


I've tried searching in the past to find the stock Cd for this car but I've had no luck.

Speaking of not knowing what the word "lug" means, I was surprised when I found out. I suspect that even your 4 cylinder Cavalier wasn't actually lugging, just being a little growly.
https://www.gassavers.org/showpost.ph...0&postcount=13

I get better mpg at 65 mph than 60 on my 99 Grand Caravan Sport. My 99 HX M/T seems to get the same mpg at 65 and 70 mph too.

the odd thing is the only way i can go faster is to depress the gas pedal further, which would seen to indicate more use of gas, and a corresponding decrease in gas millage. i think airplanes get better millage just on the other side of the speed of sound, mayby it has something do do with that? huh.

You forgot to figure in the increase in speed. Sure, if you depress the gas pedal further and don't go any faster, then the more gas means less mileage; but if you're getting more speed for the extra gas, then it may or may not be better.

Taken to an extreme, you could idle along at 5 or 10 mph, using the least fuel your engine can use while still running, but you'll probably get better mileage at 30mph.

Oh, so there is a speed (or time) component to miles per gallon. I did not know that.

1st 15mph headwind is considerable
2nd averages do not represent the same route with speed as the only variable.

test is inconclusive.

Yes. If you do some "dimensional analysis" you can conclude...

speed (mi/h)/volumetric fuel flow rate (gal/h)= mi/gal

If you think about it a little bit, it makes sense. Lets say for the sake of argument that your car burns .5 gal/h at 2200 rpm. If you compare lower speed (i.e. 2200 rpm in low gear) with high speed (2200 rpm in high gear) you will see that the mileage is impacted pretty significantly.

Which also makes it easy to see if you can maximize the time your engine is idling (using little fuel) while simulatenously maximizing distance travelled, P&G is a great way to improve mileage.

Silly me! All this time i thought mpg (miles per gallon) had to do with how many miles my car went on a gallon of gas! So under some circumstances, I can go more miles per gallon if i go faster?

Sorry dude. Wasn't trying to be a prick or anything.

Well, you seem to have answered your own question. Not sure I have much more to add.

Haha, it doesn't have to be too hard for anybody to understand these things. I just like to know the basics and use a little common sense as well. My gallons per mile and miles per gallon and gallons per hour are all so interesting...:rolleyes:

I think I am starting to understand. miler per gallon is not simply gallon/miles = mpg.

it is really:

speed (mi/h)/volumetric fuel flow rate (gal/h)= mi/gal

You're on the right track by using mathemtical formulas, they help reduce the chance for miscommunication.

MPG really is miles/gallon. That much is true.

Distance = speed * time.
Miles = speed * time.
miles/gallon = speed * time / gallon
Or, if you move "time" to the other side of the equation (because "volumetric fuel flow rate" is GPH = Gallons Per Hour = gallons / hour)
miles/gallon = speed * gallons / hour
Now, because "speed" is miles per hour (miles / hour), when you expand the equation..
miles/gallon = miles / hour * gallons / hour
which simplifies (if I remember algebra correctly, and I probably don't) to
miles / gallon = miles / gallon


You can measure the distance, divide it by fuel used, and get average MPG for your trip.

You can measure speed and multiply it by fuel rate and get your instant MPG.

You can take that instant MPG on a continuous basis and average it out and get the same average MPG as when you divide total distance by total fuel used.

Or we could treat distance as a displacement vector and say that if you eventually end up back where you started from you got 0 mpg. :D

I'm suprised nones mentioned the turbo and cam....both of which increase power and efficiency in the mid-high rpms. I have a buddy with a turbo cressida that would regularly see 25-28 mpg doing 80-120 mph highway trips in a car thats rated 24 mpg without a turbo.

My truck also seems to get better MPG at higher speed. I've tried going slower but my best tanks were after 75 mph runs followed by 70 mph runs followed by sub-70 mph runs.

so it would not be wrong to consider "miles per gallon" (mpg) the number of miles you can go in your car on a gallon of gas? I was starting to think MPG had something to do with speed as well. I am starting to get more confused... again. for some reason, maybe because of my watching too much television, i thought MPG = milers per gallon. I can see you have a much more complex and complete understanding of gallon/miles. Really almost philosophical!

"Miles per gallon" is indeed as simple a unit as the distance you can (or do) go on a gallon of gas, literally (how many) miles per (for each) gallon.

The whole point of this conversation is that a given car does not travel the same distance on a single gallon of gas at different speeds.

At one speed you might get (travel) 20 mpg and at another speed you might get 30 mpg.

I can see why a car with a turbo might do better at high speeds, at higher power levels you are running the engine as a combined otto and brayton cycle machine which would increase the efficiency considerably according to theory.

Well, in the end the point is how many miles your car has travelled against how much gas was used.

The point is that the car can get different mileage depending on its speed. This is why the EPA has "city" and "highway" ratings.

The comparison is as I mentioned before. At some RPM, the engine uses a certain amount of fuel.

During city driving, the engine is running at that RPM but in low gear, so the car travels a short distance.

At highway speeds, the engine is running at that RPM but is in high(er) gear, so the car travels a larger distance than it would in low gear.

Would an example calcualtion give you a better idea of what is going on?

I think I get it. MPG is unrelated to how long it takes you to get there. It is also unrelated to how fast you are going. MPG is simply, Gallon of fuel/miles traveled = MPG. There are vast number other equations with a nearly infinite number of variables that can be plugged in in order to learn all sorts of things, but MPG is milers per gallon. Whew!!

During this highway trip, I only got into boost on a copuple of occasionas and it was only 1 or 2 psi since I have my 87 octane tune loaded on the computer. But now that you mention about the cam, it does start making a good increase in power at 3,000 rpm. At 2900 rpm the car is going 76 mph so it's just starting to get into the power band of the camshaft.

Hmm, I need to do some aero mods and see what kind of gas mileage the car will get at 80 mph!! :D

Its a complex issue, how fast and efficiently can you go before the aero dynamic drag eats you fuel up, are you driving with or against the wind, air temperature, engine timing, valve timing engine load, road gradient and so on, on my car the speedo has a notch at 56mph to 64mph, its suggesting that this is the most efficient speed for car, probably based on the engines lowish torque and tall gear ratio and the fact aero dynamics get viscous above 60mph, I think the power required increase by the square root?

I can do around 70mph and get decent FE wiith 16Hg vacuum, I can do 80mph and the FE dips by 10%, I have done 140mph with 14psi boost for 10 miles and I swear I saw my fuel gauge move down like the minute hand on a big clock, slower is generally better but I would say you need to know the peak torque band of your engine and its most efficient fuel range, because it depends on the load and how long the injectors are open for.

Actually, required power theoretically increases by the square of speed. This is, of course, used by SO many advocates of lower speed limits to make it seem like slower is always better. BUT, there are other factors to take into account. One of them is drag coefficient. This is often treated like it is some sort of constant. But it is NOT. Drag coefficient can basically do ANYTHING as you speed up - it can increase, stay constant, or even decrease. Pretty much ANY engineering text has some sort of a plot of drag coefficients for various geometries vs Reynold's Number (a dimensionless quantity which takes into account the fluid, size of the object, and speed). And there is virtually NOTHING predictable about the curves. Sometimes, a sharp drop can be seen at some point. Because of this, it is VERY possible that a given car might experience LOWER aerodynamic drag at higher speeds rather than the higher drag that everybody thinks is the general rule.

Of course, there is also the factor that someone mentioned about engine efficiency. Some engines just don't like low revs. For instance, when I tried to use a CRX HF transmission on a DPFI Civic DX, I was rewarded for my efforts with a car that was REALLY good at ridding me of all that pesky gas that was in the tank. I would guess that this whole 'taller is better' thing all goes back to the old days when all engines had only two valves per cylinder and a 6500RPM redline was considered sky-high. However, nothing these days has two valves per cylinder other than GM V8s.

I guess this all means that you have to experiment to find out what speed works best when it comes to mileage. Because if you just stick firmly to the 'slower is better' mantra that is just SO common in fuel economy circles, you could find yourself burning MORE gas in order to take MORE time to get where you are going. And that certainly doesn't do you any good, does it?

You say DPFI civic DX, why so vague? How about you be more specific about the car you're talking about. Also while I have read that the civic HF has taller gears than the VX, I've also read that the VX has taller gears than the HF so I have no idea who to believe. Generally it has been observed that a taller gear ratio results in better fuel economy. I think what happened with you and again I have no idea what civic you're talking about, but I think you simply reved much higher than before because of the taller gearing and or you drove around in WOT which caused the ECU to dump a bunch of fuel, again I have no idea what civic you're talking about.

Be more specific about the "civic DX" since for all I know you could've put it into a civic DX of the same era.

Something just jumped out and poked me in the eye, don't know why I didn't see it before when I was playing with these numbers, see last half of this post...

https://www.gassavers.org/showpost.ph...3&postcount=47

Now it makes sense why a lot of people with minivans are saying that mileage is better at 65 than 55. I feel something on mine around there, I thought it was the motor getting more fuel or something, but it appears I'm transitioning flow regimes and getting a drop in drag.

There's a funny thing here... if your vehicle is very unaerodynamic, then you maybe get a jump in drag when flow transitions, so 55 mph is your speed. If it's mildly aerodynamic, you transition somewhere in the 60-65mph range... if it's very aerodynamic, you transition around 70-75mph, as in all those Corvette owners saying mileage is best at around there. Of course it climbs again with the square of the speed still, but it appears that because of the effective Cd change, you get a notch in the envelope.

Dammit, now I know what that is, I'm not staying at 55.

Having an engine running at its most balance speed is important as well, blue printed engines seem to give better FE, there is always a perfect balanced engine speed, knowing that may help your FE as well, less energy is wasted shaking the hell out of 400 pound of engine!

Aha, coast (CIG) tested at 110-100Kph and 100-90kph.. 110-100 = about 17 seconds 100-90 about 12 seconds. I knew there was a "sweet spot" there but didn't know if it was the ECU increasing fuelling, the throttle cam radius tightening, or what.... but it appears to be the aero... trouble is trying to keep it at 105ish, merest twitch of the foot has it racing away up to 115ish.

Very interesting results!! I'll have to try that test at differant speeds in Wagonstein to see what the results are.

DPFI = dual point fuel injection. This could only be a 1988-1991 Civic DX or STD. In my case, the car was actually a STD with a DX motor and ECU. And no, it had nothing to do with revving the motor during shifts. We are talking about freeway mileage here. And the car just didn't like that HF transmission. It got something pathetic like 27-28MPG on a drive to Portland from Sacramento earlier this year. That's down a couple from what the car used to get with a 1988-1991 SI transmission. And my B18C5 CRX would have done MUCH better than that - and been able to climb the passes better.

As for gearing between the VX and HF transmissions, they are about the same. And differences in gearing are pretty minor.

How many HP + torque is that engine? I bet the problem was that the car was too heavy and underpowered for a transmission such as the HF.

The engine has 92HP and 89ft-lb torque. SO it's not a powerhouse. But it has more HP than the HF motor (62HP, 90ft-lb torque). And the car only weighs 1933 pounds. This, by the way, it LIGHTER than a 1990-1991 CRX HF (although it is certainly less aerodynamic).

I did some coasting testing yesterday. At 70 mph, coasting down 5 mph to 65 mph it took 10 seconds. At 60 mph coasting down to 55 mph it took 11 seconds. So it seems like the aerodynamics are a bit worse at the higher speed, but not by much. I want to do this again, but test at 75 mph and at 65 mph so I can get a better idea of how the aerodynamics are at multiple differant speeds.

Measure distance in the coastdown test, not time. You measured 1/10 time shorter coast for the faster speed, but you were going 1/7 faster, which I think means you coasted a longer distance (which is what matters for FE).