rv gas mileage

jplante4 wrote:

wa8yxm wrote:
Major differnce is the Triton, being a Ford, is a HIGH REV engine, producing the best power curve at a much higher RPM than the low-rev GM Vortec.

This may affect engine life,. but not so much MPG.

Physics says this isn't so. A higher RPM should burn more fuel. 460 cu in of air/fuel mixture going through the motor for every RPM, 15:1 air to fuel mixture indicates that higher RPM engines will use more fuel.

I had to explain this to a co-worker who bought her first car with a manual transmission. Her father told her to keep the revs high as possible. I explained that a lower RPM is fine. When she took mt advise, she commented that she was using less gas.

That's an oversimplified analysis. You're assuming that the same amount of air/fuel mixture gets into the cylinder each revolution, which is not true--the throttle valve alters the size of the charge. (Incidentally, the displacement of a 6.8 L engine is not 460 cu in, but rather 415 cu in.)

In practice, for any given engine and a constant power output, the efficiency should be better at lower RPMs and coincident higher throttle settings, at least in general terms. This is because the total internal friction of the engine is lower at lower RPMs and the pumping losses are less at higher throttle settings, both of which mean the engine is more efficient at converting the chemical energy of the fuel into useful mechanical output.

However, particularly at greater than marginal power outputs, the fuel usage more closely follows power output than engine RPM as the changes in engine efficiency tend to be comparatively small.