Fuel efficiency better at higher speeds?


#1

I drive a '92 Buick Skylark, 2-door, w/ a 2.3L 4 Cylinder 120 hp engine 3 Speed Automatic Transmission, A/C broke long ago.



Occasionally, I find cause to take a road trip and keep track of my fuel efficiency and average speed along the way. I recently set a new personal record of 383 miles on a single tank of gas (~14.2 gallons), averaging ~27 mpg at nearly 75 mph.



Over the years, I have noticed a trend contrary to the popular opinion that slowing down increases fuel efficiency. I find that the faster I drive, the more fuel efficient my car becomes. It seems to me that my car is most efficient at speeds well in excess of 80 mph. Could this really be true, or am I discounting certain variables?


#2

I am far from an automotive engineer, but that really does not sound right to me. Perhaps someone with greater knowledge will have a different opinion.

There are things in the design of an engine that can give better mileage at slightly higher speeds, but the Brothers have said usually best mileage will be at the lowest speed at which your car will shift into the highest gear or overdrive (assuming automatic transmission.)

For example on my Sienna, it will lock up the OD at around 40-45 mph.

By memory here, correct me folks, the losses due to air speed go up four times as your speed doubles. In any case, air resistance goes up as the speed increases, thus air resistance at 80 mph will be much, much higher than at 40. So, even if for some reason your motor is set up to get better performance at higher rpm, the air resistance should be much higher.

Do be aware that not all gas pump shutoffs will work exactly the same, so on one trip, mileage can very dramatically because the fill came out differently. I see a lot of variation on long trips, between fills, but over a 1500 mile trip, with three or four fills, I can see an average.

I had a, I think it was, 1985 Pontiac big station wagon, and a guy I worked with told me I should be getting 23 mpg at 70 mph on the highway. He claimed he did, and I did not believe a word of it. Short fill, since that heavy old car could never get that good mileage. I drove it too much to conclude I was driving lousy to be only getting 17 mpg.

Having said that, I had to rent a Buick Le Sabre 8 or ten years ago, after getting rear-ended in Austin. We drove 1100 miles to Iowa, and got over 30 mpg at highway speeds, which I found out other people did as well. An Amazing engine/tranny combo. But, I would expect it to do even better at 50 mph steady.


#3

This is hard to believe at 75 mph +. Road/wind resistance should be causing a major drop in economy at those kinds of speeds. And it’s more than opinion, the measured tests all back up the ‘go slower to save gas’ advice.


#4

For your own vehicle this is nice to learn and it’s the only way to ever answer the question. One vehicle at a time, where is the efficiency “sweet spot” ? These posts have argued this point before…telling me I’m full of *it when I told them my personal results concerning just one of my trucks. I have two others for which slower is more fuel efficient ( 79 chev c10 350 eng/thm350 3spd auto & '06 Escape hybrid ) but I found your experiecnce to be true on my '92 Explorer within the narrow perameters of speed and time for which I made the calculations. No, you’re not nuts, you’ve just learned your car’s attributes and the theory does not apply accross the board.


#5

I think the situation is that you get better mileage in situations in which you are able to drive at higher speeds. Being able to go 75+ requires light traffic with no stops and steady speed, which is good for gas mileage and in a 4-cylinder Skylark, you probably also need favorable winds and a smooth level road surface to cruise comfortably at that speed. I’ll bet that in the same situation, if you could keep it at 65 you should be able to break 30mpg easy.


#6

It’s possible. A lot depends on the engine, torque, and the power band. Some engines will be at their best in a certain part of that power band. Think of it as a “sweet spot”. It varies by car but sometimes an engine can nearly be lugging at a lower speed. It depends on the car and engine, etc.

I have a Lincoln Mark VIII with the DOHC 4.6 V-8. That car gets better mileage at 70 MPH than it does at 55. (About 1.5-2 MPG better and it’s been verified a number of times.)


#7

You’re not calculating things right. There’s no way you can drive faster and achieve BETTER gas mileage. It has nothing to do with popular opinion. It has to do with physics. Wind resistance is the square of your speed. If you travel at 50 the square would be 2500. If you travel at 60 the square would be 3600…that’s a 25% increase in air resistance. At 70 it would be 4900…that’s almost a 100% increase in resistance.


#8

here we go again.


#9

Headwind or Tailwind? I own a 2001 BMW 7-series (with the V-12) and on a recent 120-mile all highway trip here in Iowa, I achieved 27.8 mpg heading to my destination, but only 22.4 mpg on the way home. Reason: strong tailwind on the way out, same wind on the way back but I was driving into it rather than it pushing me.


#10

I think the situation is that you get better mileage in situations in which you are able to drive at higher speeds. Being able to go 75+ requires light traffic with no stops and steady speed, which is good for gas mileage…

GreasyJack has it right. If you could duplicate these driving conditions and drive 45-50, your fuel economy would be even higher.


#11

I think one factor everyone seems to forget is the time the engine is running. As you go faster, you reach your destination sooner, so the engine is running for less time. There seems to be a breaking point where if you go fast enough and reach your destination soon enough, the decrease in driving time makes up for the increased fuel consumption.

I’ve found the same thing driving to see family halfway across Ontario. I’ve gotten better mileage going 125km/h and arriving an hour earlier than going 105 or 110. I attribute the decrease in consumption to decreased driving time, because nothing else makes sense.


#12

Actually, the “engine running for a shorter time” is what doesn’t make sense. This was debunked very nicely in a thread a few weeks ago. Perhaps the person who did so can explain it again. The explanation was quite eloquent, and I won’t try to re-create it.


#13

As an environmentalist you should be aware of the basic laws of nature. All your fellow environmentalists will espouse the fact that air resistance increases with the square of the vehicle speed, and rolling resistance increases even more! So, the lowest speed i the highest gear generally gives the best mileage. Just get on a bike and go fast to see how much more effort it takes.

I drive to a city 180 miles away several times a month. Even tanking from the same pump, my mileage is all over the map, so I take an average of several trips. The best mileage I ever got was a long weekednd trip back from the mountains with very slow (45 mph) traffic, but very few stops. The mileage during this trip was nearly 10 mpg BETTER THAN MY YUSUAL 75-80 MPH highway driving.

Whatever you do, don’t rely on that mileage indicator on the dash; it only indicates less mpg as you speed up. The figure is not actual.

We get a fuel mileage post every week, and as others have stated, gas mileage has many variables, and unless you duplicate the exact route with the same weather conditions, and pump gas from the same pump, you are fooling yourself with the mileage results you mention.

You are the first poster who actually claims a significant increase at high speed. The whole science of aerodynamics deals with reducing drag at high speed so as to get better mileage. So take our word for it, and do a number of tests yourself and allow for different driving conditions.


#14

If the engine was running at a constant speed, the idea that the engine is running for less time would hold water. However, when you drive, the engine changes speeds, and fuel use fluctuates. In order for you to know this one factor affects fuel economy, all other factors would have to be equal. In the real world, all the other factors are not equal.

Environmentalist, I am pleased you understand your fuel economy calculations are estimates. People often get the idea they can measure miles per gallon precisely over the course of a single tank of gas. Where your logic fails is that in order to test your hypothesis, you would need to get precise measurements in an environment where variables can be eliminated (or unmeasured factors can be equalized). You would also need to fill up at the same gasoline pump, using the same nozzle each time you fill up. Based on what we know at this point, the cause could be any number of unknown factors.

Slowing down doesn’t necessarily increase fuel economy. The increase in fuel usage that most people see when driving in city traffic is related to stopping and accelerating repeatedly. The main savings you see in fuel economy from driving on the highway can be attributed to the fact that you are traveling at a relatively constant speed, regardless of what that speed is. Looking at wind resistance and how increased speed uses more fuel, it really depends on how aerodynamic the car is, the weather conditions, whether or not you travel at a constant speed, your tire pressure, etc. It could be that maintaining a constant speed is easier when you drive faster. Is maintaining a constant speed is harder at slower speeds? How is that for a theory? Do you use cruise control? That would eliminate the “speed variation” variable.


#15

Quit with the “science”, Quit with the “math”…go drive my truck ! Never was it stated the ever and ever faster would result in less fuel consumption…never said that. But as for the conditions for Environmentalist and for myself , in my one truck , 82mph vs 70mph 300m round trip, I, ( JUST ME ) put in less fuel for the total trip when cruising at 82mph. 15 year old truck has repeated these citeria time after time. I know my truck . NEVER NEVER NEVER DID I SAY THAT THIS WOULD EVER BE TRUE FOR ALL CARS OR FOR ALL SPEED DIFFERENCES. STOP ARGUING WITH US AND GO DRIVE MY TRUCK.


#16

You can say all you want…I don’t believe it.


#17

Oh, rest assured, I know that going much slower will garner better mpg ( ie; just above the speed that overdrive shifts in, say maybe 55 ) but I just cant snooze my way to Albuquerque for nearly 3 hours that this would result in. Also, this particular truck is horrible in town so every little advantage I can find for it…I’ll take it.


#18

Please…you’re giving “real Environmentalist” a bad name. Tell me you’re pulling our leg !


#19

I’ve calculated it two ways, and at the same time. One is with the onboard readout (which is pretty accurate) which was used in conjunction with the “fill the tank/trip meter” method.
It always matches up and I’ve checked this more than a dozen times. Matter of fact, the first part of next month I’m off to CO for a week and will be checking this again.
Going up there will involve the slower, unobstructed 2 lane roads before hitting I-70 in KS.

I’m a stickler for mileage mainly because it lets me know how the car is doing performance wise. Sometimes engine torque is a good thing to have and makes wind resistance almost a non-issue.
The car has 2:73 rear end gears and at 55 MPH the engine is near lugging since it’s only turning about a 1000 RPM. Bump it up to 70 and it’s a different beast.


#20

If one takes a small 4-cylinder manual transmission car and drives it at 30 MPH on highway while in 5th gear does this mean the gas mileage will be dramatically better than if one were doing 50 MPH in the same car while in 5th gear?

Do power curves and lugging mean nothing?
Just curious to hear the response on this is all.