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# Gau Mar 11, 2008 3:16 PM GMT Actually no, 55 isn't the most efficient speed The best ratio for gas is just above minimum rpm in your highest gear. RPMs is what eats gas, not speed... t Generally older cars run from 45-60 depending on gears and ratio. However on a lot of newer higher end cars with 5 and 6 speed gears, those number goes up...
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Both RPM and speed eat gas, and by that logic alone, the most efficient speed is 1mph. However, the BSFC map changes all this. The most efficient speed is where you can go fast enough to get into a good spot on the BSFC map, but not high enough that drag eats up all your efficiency. If you drive looking only at the BSFC map, you'll be getting the most crank power from your gas, but you'll be literally throwing it to the wind in the form of air resistance. We're going for MPG, not HP/G, so the only way to really find out is to test it out. My automatic 2.2L Cavalier peaks at 42mpg @ 53mph. There's another member on here with that powertrain who might find that info useful. For other powertrains, it will differ.
On the slow end, gearing limitations and the low end of the BSFC eat your MPG. On the fast end, wind drag does.