The Speedometer in my truck is pretty inaccurate and I am trying to get an idea of how much.  My truck now has an electronic tach.  So this would be easy if the truck was manual shift but it isn't, it is a Ford-O-Matic. The tires on the rear (235R75-15) have a rolling, loaded, circumference of 90.69" and the revs/mile are 720.  So with the rear axle ratio of 3.73:1 the engine speed with no slippage will be 2685 RPM @ a true 60 mph.  What would be a reasonable assumption for slippage in the torque converter at normal load and at W.O.T. ?

Of course I could run down the road with a second car at the same speed and answer this question for normal load.

Doug T

The Highlands, Louisville, Ky.