But it is indeed just that simple!!! Again, you are guessing, and you need to collect more facts before any conclusion can be drawn. If your engine is really turning 2000RPM at 60MPH, and seems to be relatively fuel-efficient at that speed, my best guess would be that it would be even MORE fuel-efficient at a slightly higher speed. That's my guess, as I really doubt any engine would have a 40% speed of 2000RPM. For that to be true, the engine would need to have a max speed of 5000RPM, and the tachometer would likely be marked to discourage you from going beyond 4000RPM. That's really slow.
As I posted before, in your car, it's a good bet that 60MPH is closer to 40% than 80MPH. But unless you can post the max RPM for your car, there is no way to determine where the 40% range is. 60MPH-2000RPM might be 30% and 80MPH might be 60%, and BOTH might be outside of optimum fuel-efficiency range.
This is exactly why I said you should never buy a car without a tachometer. With a tach, you can do the math to get a damn good estimate of where the engine's most fuel-efficient RPM might be. WITHOUT a tach, you will have to find the most fuel-efficient point by lots of trial and error. In your case, 60MPH is fairly fuel-efficient and 80MPH is somewhat less so. Where's the point of greatest fuel efficiency? Neither one of us knows. You just have two data points out of dozens of possible data points. -Dave
