Trimatic overdrive

[axistr]

You need to calculate for any slip difference from the torque converter. At 100kph there might be 300-500rpm difference between the engine rpm and trimatic transmission output shaft. In later transmissions like the 4l60E they lock up the torque converter to give it a so called overdrive. In a lot of autos its not actually an overdrive ratio only a direct drive speed (engine and transmission speed the same) Its the converter lockup that allows lower engine rpm for better economy.

Its always better to jack up the vehicle with both wheels off the ground, use a marker on both wheels and floor, pinion and housing then rotate the pinion till both wheels have done exactly one complete turn and count the pinion turns. Excessive back lash in the hemisphere gears may give you an inaccurate reading doing one wheel only and double the readings.