Automobile - Calculate the Torque of Propeller Shaft

Going uphill or down?
How much headwind/tailwind/drag coefficient?
How heavy is the car plus payload?
How well inflated are the tires?
Somehow T = KP/n doesn't quite seem to capture all this....

First you must know the tractive force required. As Tornado writes, this depends upon air resistance, grade, and tire rolling resistance. Then, it is a simple matter to calculate propeller shaft* torque given the tire radius and the final drive reduction. (The losses in the final drive are probably small enough to be ignored for your purposes.)

The air resistance can be calculated if you know the frontal area, the coefficient of drag, and the speed. Grade causes an additional tractive force requirement equal to the vehicle weight times the grade in percent (for most small grades of probable interest). If you don't have a better figure, tire rolling resistance can be estimated to be 1% of the vehicle weight, and doesn't vary much with speed.

Another approach, (from the other direction, in a way) would entail knowing the hp required to travel at 60 k/h in the car in question. This could be deduced from fuel efficiency at that speed, and some fudge factors. (An average car would require about 10 hp to maintain 60 k/h on a level road.) The hp at the drive wheels will be very close to that at the engine (and for your purposes you can probably ignore the transmission losses, which are about 2% each in the transmission and in the final drive). Given a drive wheel hp and drive wheel rpm, it is simple to calculate torque at the drive wheel (and anywhere upstream, based on the reduction ratios).

* Here, I am using "propeller shaft" to mean the conventional drive shaft between the output of the transmission and the input to the differential.