# Different gas mileage at different speeds

I heard that most new cars are the most fuel-efficient when you are travelling at 55 MPH. I also heard that if you go faster than 55 MPH, you Miles Per Gallon ratio goes down. Is this all true or not?

Depends on gear ratio. For example, my cherokee runs at about 2200 rpm in third gear at 45mph. Fourth gear takes me to near 60mph at 2000 rpm. 70mph requires another 600-700 rpm. More rpm = higher gas consumption.

Near as I can tell, most cars sold in America are geared this way to comply with speed limits.

I know that in the '70s cars were made to have optimum mileage at that speed. That is why the speed limit was made 55 - nothing to do with safety, but simple energy conservation.

My guess (and it is only a guess) is that cars all have slight variances with this, and that newer cars are getting optimum mileage at higher speeds because of the speed limit being raised.

It would make sense anyway, but you know, we are talking about the auto industry…

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Brother Haus has heard: The faster you drive your vehicle the more gas you use. So, your gas mileage actually goes down the faster you drive. I could be wrong, but it does make some sense to me.
=Brother Haus goes the speed limit=

I am not weird, I’m just normle challenged.

Louie, I don’t know if 55 is the optimum economy speed or not.It sounds kind of high.Sly, is right about it having to do with the rpm( thats a nice sound,pronounced rupum to make when you are sitting stil and reving the engine, "RPM, RPM,RPMRPM,RPM!) It also has to do with the timing, you want the spark at the optimum time,as your speed increases the distributer advances the spark. Actually most cars aren’t driven at highwayspeeds,the engineers had to make a compromise,you gotta keep the car running at idle, have it run eficiently at ‘normal’ speeds but not have it guzzle to much at highway speed.
Now away back when I was terrorizing blue haired ladies, the speed limits were at 60 on th high ways. The optimum economy speed was around 45.Course now with all this new fangled puter stuff and fewell injectors it all may be moot.And all that stuff makes it way hard to even find out whats wrong with the car,much less fix it! BTW I got these lil pills you can drop in a tank of water and drive for miles, Email for details.

“Pardon me while I have a strange interlude.”-Marx

As you press the accelerator harder, more gas is involved in each little explosion inside the cylinders. So driving faster does use more gas.

However, the number of explosions per second is based on the engine speed, in RPM. So if you keep the gas pedal in the same spot, and shift to a higher gear, you’ll suddenly be burning less gas.

According to the Car Talk guys, if fuel economy is your only concern, you’ll want to drive in your car’s highest gear, as slowly as possible.

Of course I don’t fit in; I’m part of a better puzzle.

I’m picturing Aura tooling down the autobahn in 5th gear at 40 kilometers an hour.

You people seem concerned only with how hard the engine is working. Fuel economy also involves how long the engine is working. If you work the engine harder, but get to your destination fast enough, you could be ahead of the game.

Boy…back to undergraudate school:

As I recall, the greatest factor in fuel economy assuming engines and machines are well designed and as efficient as possible already, has to do with the drag produced by the vehicle travelling through air.

First, lets look at drag. See this NASA link:
http://www.grc.nasa.gov/WWW/IFMD/airplane/drageq.html

You can see that the largest effect on drag aside form things like cross-sectional area is the velocity of the vehicle. In fact, drag increases as the square of the velocity!

Next, lets look at the SHAPE of the object and its effect on drag coefficient:
http://www.grc.nasa.gov/WWW/IFMD/airplane/shaped.html

It is interesting that a BULLET, which is what I consider more like a car, has one of the highest drag coefficient.

When you put these, and other factors togather, you will find generally that drag will increase as speed increases for a given shaped body travelling through air. In fact, as I recall, drag DRAMATICALLY increases somewhere around 60mph. Now I do not have the papers anymore with the experimental results to show this, maybe someone reading this can find it.

This sudden rapid rise in drag is due to factors like the dir flow becoming turbulent (air separates from the surface laminar flows). This is the primary reason why back in the energy crisis, the national speed limit is set at 55mph - your gas mileage will drop dramatically as the the drag rises dramatically.

It is interesting that even for speed skaters, someone has gone to the trouble to analyze this (30mph):
http://home1.gte.net/pjbemail/dragcoef.html

Incidentally, the NASA site has some pretty good introductions to these subjects. Its good to see our tax money not totally wasted.

Presuming that you stay in the same gear, driving faster will always be less efficient, because you’re encountering more air resistance. Specifically, the air’s frictional force is proportional to the square of your car’s velocity; every time you double your speed, you quadruple the amount of air resistance.

So air resistance can be ignored at low velocity, but it becomes quite important at anywhere near normal driving speeds.

Of course I don’t fit in; I’m part of a better puzzle.

Dangit, I hate getting beaten by a better answer.

Err…I meant to say that the bullet has one of the LOWEST Cd. Cars are well designed these days.

one thing people have overlooked, is that engines operate at different efficiencies (sp?) depending on RPM. A dragster engine idles at 5500 rpm and produces max power well above that. A regular car engine would not produce any power at that point- all of the power would be fighting friction (air/fuel/exhaust resistance, engine bearing drag, etc.) This difference is caused by multiple factors, ignition timing, fuel type, size/diameter/number of intake runners, the number of valves per cylinder, camshaft timing, valve lash (the measurement of the “slack” in the valvetrain, valve geometry, and a zillion (technical term) other things. The upshot of is- one engine may get best milage in the same gear (same trans/final drive ratios) at 2300 rpm, and another may get better milage at 1300 rpm and still have the same displacement, bore, stroke and design. There are a lot of factors not even directly connected to the engine, such as stall speed in the torque converter (the speed the “clutch” of an auto transmission starts to “grab”), tire size, diameter, weight, and inflation pressure, newness of ignition components, and type of spark plug. Overall, you tend to do better by maintaining your car fastidiously, changing oil religiously, and giving your car a tuneup when needed. Not running the air conditioner helps too.

Wait a minute. Maybe at 50mph you use twice as much gas as 25mph but you get there in half the time, thus, should not they be equal?

Aura (and Click and Clack) are right:

I’ll set myself up as an expert on this question, having obtained a BS in mechanical engineering, having survived Dr. Heaton’s internal combustion engines course, and having worked for an engine manufacturer for a short time.

Your fuel economy is a combination of how efficiently your engine is operating (power delivered per gallon of fuel) and how hard it’s working (power required to move at that speed). For internal combustion engines the fuel efficiency is higher at lower RPMs. For any vehicle moving through the air the power required is proportional, as noted above, to the square of the velocity.

So go slow to keep the power requirement down and drive in high gear to keep the engine turning slowly.

handy – if the power required were linear with speed you could get away with the 25/50 thing but it’s not. You will get, for instance, 20 mpg at 25 mph and only 15 mpg at 50 mph. So if you go 50 miles at 25 mph it will take two hours and consume 2.5 gallons of fuel. At 50 mph it will take one hour and consume 3.3 gallons of fuel.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

It is certainly true that if you travel half the time but using twice as much gas for the duration, your trip will come out even. But we are talking about drag above and beyond normal when you reach 60mph or more. You will have to use additional gas to overcome it.

Actually, the greatest factor in the gas mileage of a car is its weight. The heavier, the more energy consumed in transporting it from one point to another, thus more gas used. But its shape and drag is VERY important.

I know that when I drive my DODGE van, it makes a great difference in gas mileage if I am below 60mph than if I am above 60mph. With a 30 gallon tank, I can get over 450 miles cruising at 55mph. But jack it up to 75mph, and I will be lucky if I get 350 miles. If I load it up, the gas mileage drops dramatically also. Of course, a Farrari will slip through the air quite a bit better, but that 60mph thing still holds true.

Unfortunately, you can only decrease the weight of the car just so much before it becomes totally unsafe. And you can only shape the body just so much before no person can fit into it.

It is also true that if you accelerate S.L.O.O.O.W.L.Y, and drive S.L.O.O.O.W.L.Y, you will be get the best mileage, but when was the last time outside of Florida have you seen that happen?

The term you guys are looking for in engine effeciency is called Volumetric Efficiency. Volumetric Efficiency is referring to the amount of air/fuel mixture that is actually drawn into the engine compared to the actual displacement of the engine. Volumetric efficiency of an engine will drop as the engine RPM increases.

I have a friend who has Ford Crew Cab One Ton Dually. He says it gets the same mileage whether it is empty or loaded down and pulling his fifth wheel. What is going on there? He has nothing to gain or lose in lying about this. He told me this when I asked him how much he got when pulling his trailer. Oh yeah, it is a diesel with an aftermarket turbo.

I told you guys I was an expert. Now stop disagreeing with me!!

Volumetric efficiency is only one aspect of an engine’s efficiency. You are correct that it does generally go down as the engine turns faster and IIRC, at a certain point the change in volumetric efficiency with speed is the dominant factor in the change in overall efficiency with speed.

The usual engineering measure of engine efficiency is the specific fuel consumption, that is, the amount of fuel used divided by the power generated. Reducing the volumetric efficiency raises the sfc (lower sfc is better, i.e. more efficient) but other factors play a role too, friction losses, parasitic losses, air-fuel ratio, timing, etc.

The weight of a vehicle does play an important role in fuel economy in two different ways.

First it increases the “rolling resistance” of the vehicle – that is the friction associated with moving the vehicle along the ground. It includes things like friction at the wheel bearings and friction due to tires flexing. Rolling resistance is not highly velocity dependent. It is more or less constant above a certain speed. It is highly dependent on the vehicle’s weight, as noted above.

Secondly the weight of the vehicle is very important when accelerating and climbing hills. It just takes more work to move a larger mass. On a straight level road with no stops it wouldn’t matter much. Going down a hill you get a little bit back (a heavier vehicle will have more energy at the bottom of the hill) but anytime you use your brakes the advantage is lost.

The difference between the two posts – one had a drastic change in fuel economy with weight and the other had little or none – is probably due to different driving conditions and different amounts of rolling resistance.

## p.s. To reduce your rolling resistance and improve your fuel economy you can inflate your tires more – trade a smooth ride for economy. (You can overdo this too, of course – overinflated tires don’t have as much traction.) I think Cecil did a column on this once.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

About the only thing I can add or suggest is that even if the Ford is loaded, the truck could be so much heavier to begin with (particularly with a diesel) that the relative added weight of the payload is really not anywhere near the payload added to my DODGE B250 maxi in terms of its original weight. The DODGE is a “light” van with unibody construction and little or no frame. Asking that little 6 cylinder to accelerate with a heavy payload is like the LITTLE TRAIN THAT COULD!

For those who were not there…

Back when the limit was originally imposed, it was 50 mph. Long-distance truckers complained that they would actually get better milage at 55, and they apparently proved their case. Note that 18-wheelers are much more alike than cars are.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams