Steve,

Torque at the flywheel is obviously the same. Torque at the real wheel is
not. Torque at the rear wheel (or more accurately the thrust at the tire
contact patch) is what moves the vehicle. Torque or thrust at the road is
given by:

((Flywheel torque) x (trans gear ratio) x (final drive) / (effective wheel
radius)) - (power train losses).

For this situation, I assumed that all these terms are identical except for
the final drive ratio. Finally, horsepower is just the product of torque and
engine rpm.

The torque at the road for any gear will be reduced by the ratio (4.57/5.83
or 4.57/5.43 depending on which diesel trans he currently has. Dropping to
3rd as compared to 4th will recover thrust to the extent of the ratio of
3rd/4th.

Just facts,

Frank Grunthaner