Slow Down for Better Mileage


Here’s the rest of the story for Frank, who wants to endanger us all by increasing the speed delta on highways. At 75mph his 300mi trip would use 8.7gal (@ 44.5-10=34.5mpg). That’s 2gal more but 1hr less. So he’s earning $6/hr. Is that worth it?


He wants to increase his speed change (delta) on highways?

Sorry, Tom, but on my daily commute 75 mph would just barely keep up with the left lane…you’d have to move over regularly to let the fast cars pass. I vote that 75 mph is perfectly safe, and in fact maintains a steady traffic flow with less turbulance.

Note that this subject has generated looooooooooong threads in past years with literally hundreds of posts. There’s lots of different driving environments and lots of different perspectives on this.

  • mountainbike


Personal experience in my 2002 Hyundai Accent with 1.5l I-4 has shown that I get better gas mileage by driving faster.

At an average of around 85km/h, I’ve gotten about 43-45mpg. An average of about 100km/h has yielded a mileage of 45-50mpg. At 120km/h, I get 49-52mpg. These mileage estimates were calculated over a 7 month period, driving on roads with 80km/h speed limits (the higher speed calculations were done during a number of trips on 400-series freeways, with speed limits of 100km/h.) Mileage was calculated the usual way, trip odometer divided by amount of fuel used, which was determined by filling the gas tank up until the nozzle automatically clicked off. This was also repeated numerous times to determine an average fuel economy (I was filling up every other day for 7 months.) So, based on my experience with this one vehicle, I get better mileage at higher speeds. Of course, this is not a scientific or well-controlled experiment, but there does seem to be some correlation between increased speed and increased fuel economy.


I think it probably depends on the engine and it’s power curve. Some engines do better in higher RPMs, others it’s lower RPMs, you just gotta find that sweet spot and you’re set.


Speed is fun. “Driving”, in and of itself, is considered a worthwhile activity. Most children, on their 16th birthdays, take the day off from school and attempt to obtain a drivers license. Not because they have to drive, but because they WANT to drive, and will pay dearly for the privilege. If there were no speed limits and or no speed cops, our highways would become racetracks overnight with speeds in excess of 100 mph common on all limited access highways. SPEED is FAR more important than mileage for the vast majority of drivers. My SPEED PROVES I’m a better driver than you and my car is better than that pile of junk YOU are driving… Dying in a twisted pile of wreckage is noble and glorious. Innocent victims? Keep them off MY road!

When the gasoline finally runs out, half the world will plunge into driving withdrawal. How terrifying…But please, please! Can I still ride my Harley on weekends??


The notion that driving faster always consumes more fuel is false.
It all depends on the car, the engine, gear ratios, etc.

My Lincoln will get better fuel economy at 70 than it will at 55; all proven during a number of road trips.


I’m with you Tom.

I might add that I am more than a little skeptical of claims that a car will give higher mileage when driven faster under normal conditions.




Some of you may not believe it, but it’s a card carrying fact that some cars will attain better mileage at higher speeds.
It all depends on when that engine gets into a “zone” and is breathing well.

Mine has been verified repeatedly on a number of road trips and some guys in a custom engine application group involving big blocks have found that many of their engines will get better mileage at 2200 rpm rather than 1700.

It all depends on the power curve, etc. and yes, my readings were done by the fill and check the trip meter method; backed up by the trip computer in the car. Yes, the trip computer is accurate and matches the other method except for a goofy spell about once a year when it reads stupid for a day or two.


Two questions OK4450:

  1. Any particular way of finding that sweet spot by way of watching the RPM’s or just keep track of the mpg’s w/ different driving speeds over a period of time?
  2. Should the mpg’s per tank of gas match closely between the car’s computer and the method of dividing miles driven per the trip odometer since last fill up by the number of gallons taken to fill the tank until the automatic click off? I ask because it varies a good 1.5 to 2.5 mpg’s between the computer in my new car and what the old tried and true method shows…usually the car claims I’m getting better mileage than my calculations show.




Anyone else please feel free to give answers to my questions. Didn’t mean to exclude anyone by directing my questions to OK4450 in response to his post.


The notion that driving faster always consumes more fuel is false.

I think I know what you’re trying to say but if this statement is taken alone, it is incorrect. You will use more fuel to go faster. The measure used to gauge efficiency is miles per gallon (MPG). This takes into account distance traveled for a given quantity of gas. You’re using more fuel to go faster but it is being used more efficiently at one point along the power curve for the car.

Environmental conditions will contribute to a shift in this sweet spot.


Just wondering is all.


Well, not every car will be the same. Some cars will get worse mileage at higher speeds and some won’t. There are too many variables involved.
It’s strictly a trial and error method.

The reason why some cars may get better mileage at higher speeds is this. Imagine a car with a manual transmission going 30 MPH in 4th gear. It would no doubt seem somewhat sluggish and it is lugging quite a bit. Bump the speed up to 40 while remaining in 4th and things feel much better don’t they?

Some years back, I bought a new BMW motorcycle and this bike would get about 30-32 MPG while in 5th gear at the (cursed) mandated, 55 MPH speed limit. Bump the speed up to 70 MPH and fuel mileage would go up to 40-45 MPG.
The power band has a lot to do with things and in the case of the BMW, the power band was just coming in around 70-75 MPH.
(Power bands could be the subject of another discussion.)

The tripmeter and fill method is more accurate than a dashboard indicator. The one is my Lincoln is amazingly accurate for some reason, but it does take a spell about once a year in which is will give stupid readings for a day or sometimes two.
On average though, a dashboard indicator should not get 100% of your trust.
Maybe the engineers designed a “bump” in yours to make you feel your economy is better than it is! :slight_smile:


Ford Panthers, (Crown Vics, Grand Marq’s and Lincoln TC’s all share the same platform and 4.6 SOHC cam V8 engine. I own two examples of this fine car. Between 45 and 70, the mileage is fairly constant as this car has great aerodynamic drag numbers. My '98 CV gets 24 (it’s a cop car) and the '92 gets 26 and sometimes 28mpg. Both have the same 3.23 rear axle. Moving above 70mph, the mileage drops off quickly as that wall of air pushes back harder and harder.

During the “double-nickel” days, the truckers swore their rigs got better fuel mileage at 70 than they did at 55. Extensive tests were held. IN EVERY CASE, a gallon of fuel went farther at 55 than it did at 70 mph…You can’t defy the laws of physics…


To those that believe that it is possible to get better fuel mileage by going faster, I have two suggestions:

  1. Checking fuel mileage over one tank can be erroneous if you do not fill to the same level at the same station at the same pump. Filling technique is vital.

  2. Get on your bicycle and go slowly and then fast. Then decide which speed requires more effort. Those of you who can go fast more easily than slowly will also get better gas mileage going faster with a car.


Well, I have repeatedly checked my Lincoln Mark on the open road on numerous trips, including 4 trips to Colorado (about 1500 miles round trip).
The car gets about 24-25 at 55 MPH and on the open Interstate it gets 27 at 70 and above. The last trip to CO a few months ago got me 28.5 MPG.
That’s a fact and verified by actual miles and fuel used; not with a dashboard trip computer.
I also live in a rural area and that Lincoln seldom sees any city driving anyway.

My Lincoln does not have 3.23 gears either; it’s got 2:73s.


Well, I had hoped to get somewhat better mileage than from the old car and am not particularly. On the other hand, the new car is bigger and heavier and has more always on electronic gadgetry built into the vehicle than the old basic Brat Buggy.

I’m trying a trick for a few weeks. I’ve set the info readout to show the average mpg to see how it trends up and down with my driving. I seem to be getting a feel for maximizing that better than by feel and sound of the engine alone or by watching the instant mpg readings.

I have been making a practice of really paying attention to being more gentle on the gas pedal, coasting more when practical, and getting away from any jack rabbit starts. I’ll see how that plays out.

Since I am getting close to the advertised mpg ratings, albeit on the lowwwwww end of those, I can’t really holler much. I’ve just always managed to get substantially up in the high end of rated mpg on my previous two cars until the last couple years of their use.



I believe that in most cases, the people who say they get better mileage by going faster have automatic transmissions and aren’t aware of what gear they’re in.

I think that these people test at 55 MPH, and they’re only in 3rd, or their speed dictates that the car switch between 3rd and 4th most often. By pushing the car up to 65 or 75, they’re finally easing the transmission into 4th, and keeping it there! Then obviously they get better mileage.


The one is my Lincoln is amazingly accurate for some reason, but it does take a spell about once a year in which is will give stupid readings for a day or sometimes two

Interesting. I have a hunch about the reason and it is a common issue that plagues rookie engineers. The software is likely crude and a simple mathematical calculation. What they failed to do is account for overflow. No one bothered to check what would happen if the accumulated numbers exceeded the storage capacity of the allocated memory. Once the buffers recover, all is well until the next time they overflow.