Towing capacity formula

I was convinced that there had to be a formula to figure the towing cpacity of a given engine, drive train.
I found what I was looing for on a 4x4 off roading site.
The formula is E.T. x A.R.
x T.D. divided by a factor
of 2.583 E.T = engine torque A.R.= drive axle ratio T.D = tire dia.
The result will produce a figure for TOTAL COMBINED weight. This does NOT take braking and tongue weight capacity into account. This formula is strictly a way to get an idea of what
a given engine / drive train combination is capable of towing.

Jack & Gayle