What speed will give me the best gas mileage? There's got to be a formula to find out at what engine speed (tach) vs vehicle speed gives me my best MPG. Is it the exact speed at which my car shifts into the highest gear? Or a little faster? I know there's a lot of other factors, but lets just say straight and level road, constant speed. Oh yeah, and ZERO with the engine turned off is not acceptable! - BCD

I believe that the answer to your question would change with every vehicle, and every engine available for every vehicle.

I don't think a true answer is possible, but I would guess that 55 to 60 mph would be the most fuel efficient speed on average.

Yes it will, and 55 to 60 is probably right, but there's got to be a formula that will give the exact answer!

Yes it will, and 55 to 60 is probably right, but there's gotta be a formula that will give the exact answer!
Yes, I'm sure there is a formula, but not in a general sense.

No matter what the formula is, it would have to be adjusted for each type of vehicle, and for the specific engine, in each type of vehicle.

No matter what the formula is, it would have to be adjusted for each type of vehicle, and for the specific engine, in each type of vehicle.
This is true of similar vehicles, and it's even true about two of the same vehicles rollling off the assembly line, back to back.

Gas mileage can even come down to tire selection and the air pressure that you run in your tires. Most vehicles that come to my shop are running the tire pressure just a little low, and that can affect not just fuel economy, but can also sacrifice handling.

Check your tire pressure and, if it's a year or longer since you done it, have the alignment checked.

But more about the way the engine is loaded: you can listen to the engine and will find that you can hear when it's working hard. Working hard = higher fuel consumption.
Soon, you can learn what and when things cause you to need a heavier foot, and that is the point where you need to "be kind to your fuel."
I hope this helps.