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Why Gearing Works

3562 Views 17 Replies 6 Participants Last post by  Dave Whitmer improve MPG.

The short answer is engine friction.

At a constant road speed all the vehicle loads level out, but the internal drag of the engine is a variable if you are willing to play with the gearing.

Engine friction is both mechanical and fluid friction. Obviously, all those rubbing surfaces, even minimized by efficient bearings and lubrication, imposed a drag torque. also the engine is pumping around two incompressible fluids and one compressible fluid. Those too, impose their own friction torque load. In particular, the pumping friction of air goes up with the square of the engine speed.

Power is torque times RPM. The frictional torque goes up at a shallow parabola. Therefore, the frictional HP curve is cubic. Keep the engine RPM down and the frictional HP is minimized. If the "zero torque" HP is minimized then the fuel burn is minimized.
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Yep higher gears lowers the RPM's which lowers the fuel used.
That would be something to see a 7.3 doing 70 @ 1200 RPM and not going down hill.
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