Gas Mileage

[Guest]

What is the ideal RPM to run the engine at to get max freeway MPG? The way I understand it you should run at the peak torque RPM. I figure mine is about 2800 which works out to 65mph on the freeway. Below this RPM the engine isn't running at optimum efficiency and above this number it isn't getting a complete breath of air. <BR> <BR>What about wind resistance, friction etc?<BR>Any ideas?

[Dave@Moon]

That's an incredibly complex can of worms. You have to balance how close to peak efficiency you can get the engine to run with the speed, topography, wind and gearing. Any exact best rpm number would have to be conditionally related to any one of dozens of such factors.<P>In general the lower the rpm, the less air and fuel you're pumping through the system. However, if you're going uphill or upwind, you'll need more throttle at a given rpm to maintain speed and therefore run a richer mixture. Increase the wind resistance (e.g. wind the windows down), same result. Therefore the ideal rpm will vary by the grade of the raod, wind resistance, etc.<P>I had a 1980 Datsun just out of grad school that got a consistent 38 mpg hwy at 60 mph, but got a consistent 41 mpg hwy at 70 mph. Obviously it was running closer to peak efficientcy at the faster speed, even though rpm's and wind resistance was higher. <P>My future wife's 1981 Datsun's numbers for the same circumstances were 35 mpg @ 60 mph and 32 mpg @ 70 mph. Her automatic tranny was geared substantially lower than my 5 speed. So even within the same kind of car, there are huge variables to be taken into account.