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?