"If it's causing you more stress than it's worth... it's not worth it." - Yellow
Question #23148 posted on 02/20/2006 3:01 a.m.

Dear 100 Hour Board,

I appreciate the effort, but you didn't really answer my question (Board Question #22944). I'm well aware that cars are more efficient at speeds of around 55 MPH. There's a reason for the speed limit. You answered the question about fuel efficiency by speed, but I'm interested in fuel efficiency by distance. I deliberately mentioned RPMs because regardless of the speed, gas usage is ultimately directly proportional to the RPMs (it might take 3000 RPM to go 55 MPH uphill and 2000 RPM to go 55 MPH downhill).

So back to my question is: Is it more efficient to drive at higher RPMs (generally faster speeds) which uses more gas for less time, or at lower RPMs (generally lower speeds) using less fuel for a longer time. How can I calculate efficiency vs. distance for these two situations?

-Shteven the Misunderstood

PS - I have an automatic, but it does have tachometer. Don't ask me why.

A: Dear Steve,

Unfortunately the specific answer you are looking for has far too many variables (speed, grade of the hill, any wind factors, your specific car, etc ) for an answer to just be given. The ideal solution would be to do extensive testing on your car to find out the ideal engine speed that gives your car the best mileage. Then you would have to use that data combined with knowledge of your fuel economy at high RPMs to find out the effect on hills.

However, neither you nor I are going to do all of that work (as it would probably take months and many, many tanks of gas). My general suggestion would be to make an effort to maintain your cruising speed before you hit the hill (or maybe let yourself lose 5 to 10 mph through the course of the hill, not no more). It should be fine to increase your RPMs as long as you don't let them get too high. I base this answer on momentum. If you try and accelerate up the hill (thus increasing your momentum) you are using WAY too much gas. If you start slowing down too much, than you lose momentum that you already have, and you're going to have to re-gain that momentum somewhere.


PS Next time you have a very specific question you may want to provide all of the specifics, or you will force us to make assumptions again.