# Calculating most fuel efficient speed

I’ve seen many threads with posts indicating best fuel efficiency as a percentage of open throttle. To the average user, however, how do you determine this? I’ve hypothesized that each vehicle would have a speed you could maintain to achieve the highest MPG. As each vehicle, with a different shape and engine, would have a different speed, how can a driver calculate this (other than trial and error). Basically, if I knew the speed for my car was 70, and I had two highway lanes two choose from (one @ 65mph and the other @ 55mph), I would know I should choose the faster lane. I realize this is more in theory as there are a lot of other factors in MPG.

My new 2008 Corolla is not getting the mileage I’ve hoped for, and I consider myself a very efficient driver. I just don’t understand how my wife can drag race to every stoplight and get the same MPG as me (27mpg).

Thanks

I am guessing that the best gas mileage would be achieved at going the slowest possible speed that the car will go in its highest gear with the torque converter locked.

For the highway, the slowest speed without lugging in overdrive is the most efficient as americar said. No matter what speed you go in overdrive (top gear) the gearing is the same. The engine will go the same number of revolutions per mile; all that changes is the time to cover that mile. Mileage would be determined by wind resistance, which is proportional to the square of the speed. Lower wind resistance means lower fuel usage.

basically it’s close to the lowest speed you can run at (without lugging) in the highest gear. Definitely not 70 or 60 or 50mph. Gas use goes up exponentially with speed (ie, wind resistance).

Is your Corolla in good tune? Tire pressure at maximum? New oil in engine and trany? All those contribute to economy. (By Tire pressure at maximum I mean highest recommended pressure, not pressure listed on sidewall. Perhaps 35psi)

Special races are run for best mpg. The cars accellerate slowly to about 25 mph, and shut the engine off and coast to a near stop, and repeat. But I don’t think you want to do this regularly, it would be faster to walk.

[b] I’ve seen many threads with posts indicating best fuel efficiency as a percentage of open throttle.[]

Well, no. It is not that easy. While you might make a generalization and be close most of the time, it does vary.

There are several variables. Air resistance, it will be lower at lower speeds. Engine speed and engine load, vehicle weight and many other factors. Each is a trade off. The best from the point of air resistance might be 5 mph or less. But few cars are designed to run efficiently at those speeds.

``````Then just of all this is, there is no way to calculate it all easily.  Event the government does not try to do it, they run real test.

My suggestion is this.  Air resistance and acieration are the two big factors.  Max efficiency for most cars is about 40-55 mph with little difference in that range.  Slow easy acieration is sticking to slower cursing speeds will give you the results you want. Highway driving is almost always better mileage than city so driving at different times of day (traffic conditions) or longer trips than shorter. etc all enter in.

Now you say your wife gets better mileage than you.  Just how are you computing this?  Do you have two cars?  Are you using some sort of on-board computer?  Do you drive the same distance and path and traffic?``````

WOW! 3 posts written together (there were 0 posts when I started to compose) all saying the same thing!

I calculate MPG as miles traveled between refuels divided by gallons to fill the tank. I always fill the tank full and reset the tripmeter every fill up. My wife has '03 Ion coupe. Oddly she does more city driving and i do more highway.

Thanks for your reply

It should be in good tune, its got less than 1000 miles on it at the time of this writting.

Thanks for replying

Here’s a fourth one that says the same thing; stop trying to calculate; the best economy is the slowest speed in the highest gear without the engine stumbling! Drive like there is an egg between your foot and the gas pedal. I have a 2007 Corolla; it takes about 2000 miles for the car to be fully broken in; then the mileage will increase.

In town, pretend your brakes don’t work. The less you use the brakes, the better your mpg will be. Get real good at it and you gas mileage can even go “upside down” that is, your city mpg is higher than your highway mpg.

Give the engine some time to break in. I’ll bet that you see improvement by the time you get 5000 miles on it.

After 1 or 2k your engine should be broken in and you should see a little rise in fuel economy.

Every car I have owned if you go 50-55MPH in top gear gives optimal fuel mileage. Anything faster it drops by a bit and significantly past 65MPH. Slower you drive the better mileage since wind friction I believe is a squared function.

I notice this especially when its winterery out and my speeds are much lower on the highway due to traffic and safety for conditions.

What I do . . not terribly scientific . . . is drive on a flat, level, fairly straight highway . . . upshift through the gears to top gear . .6th in my RX-8 . . . and as soon as I get it into top gear, turn on the crusie control to see if the car can maintain its’ speed without laboring. You can tell. If you can’t, take the cruise off and feather the gas backwards to the point where you feel it, then note the mph and rpms. I’ve never heard the rotary “knock” or “ping” (and don’t want to push it that way. My highway mpg is almost 30 . . . a lot better than EPA estimates. My overall mpg is 26-28 all the time, unless I’m exclusively driving city or I “get on it for a good bit” The EPA city is 16 . . . highway is 22, so I’m satisfied. Rocketman

What car (including engine and transmission does you wife have and what engine and transmission do you have?