Gary,
There's a very easy way to determine just how much your spare tire's pressure will rise when it's mounted on an axle. On a cool morning jack up one wheel of your TM. Adjust the pressure of the tire on this side so that it is exactly 50 psi. Allow a few minutes for the compressed air in the tire to cool down. Then lower the tire completely and measure the pressure within it. I'll bet that you're going to find very little increase in pressure when it's resting on the ground, under the weight of the trailer.
