Drive axle ratio.
4.06 means that the differential input will turn 4.06 times as fast as the differential output.
This will have a direct effect on engine rpm and also be multiplied by the respective transmission gear ratio.
4.06 times the torque would be applied to the axles as well.
Then you factor in the actual gear ratios of the respective gears to get total torque multiplication factor.
In the case of 1st gear in the MTX-75 it would be 3.42 x 4.06 or 13.89 times the torque.
Example
350TQ engine in a 3500lb car with a 2.48 1st and 3.23 diff
8.01 x 350 / 3500 = .80 TQ per lb.
175TQ engine in a 3000lb car with a 3.42 1st and 4.06 diff
13.89 x 175 / 3000 = .81 TQ per lb.
