The formula for pressure increase in a constant volume container ( tires are pretty close to a constant volume) is:

P1/T1= P2/T2

Where P1 is the initial pressure, T1 is the initial temperature.

Where P2 is the new pressure, T2 is the new temperature.

The temp must be in degrees kelvin.

Or P2 = P1 X T2/T1

For P1 = 85 psi, T1 = 294.26' kelvin, and T2 = 305.37' kelvin. (T1 = 70'F, T2 = 90'F)

P2 = 85 X 305.37/294.26 P2 = 85 X 1.038 = 88.21

So for a 20' F increase in temp the pressure should increase by about 3 psi.

The bad news is that apparently water vapor in the air in the tire can cause the numbers to be somewhat larger than those predicted by the formula.

( Kelvin: A temperature scale whose zero point is absolute zero, the temperature of 0 entropy at which all molecular motion stops, -273.15° C. The size of a degree Kelvin is the same as the size of a degree Celsius.

K+ 'C + 273.15)