The Method Varies with Season and Cold-Tire Temperature
I like to go on a trip with a rule of thumb in my head (with simplified math) so I don't have to take out a calculator at a gas station. In deriving a rule of thumb that is easy to remember and use on the road, the simplest thing for me to remember is how many degrees of change it takes to change tire pressure by 1 PSI, reducing what I have to remember down to a single number. From that, I can quickly measure my tire temperature, do some math in my head (simpler than the formulas above), and know what pressures my tires should be. However, it is important to realize that one number does not fit all circumstances. This number is affected by BOTH your cold-tire temperature (which varies with season and location) AND the cold-tire pressure you are using for each tire, so you might have one number for your front tire and another for the rear. I have created a table below to illustrate how cold-tire pressure and temperature affect this number, and to make it easier to look up and remember while preparing for a ride or trip. (The table has been expanded to include car-tire pressure ranges, so that it is useful for a broader range of vehicles.)
How to Use:
For each tire, follow column 1 (T1) down to your cold-tire temperature, then read across to the column representing your cold-tire pressure (P1). Result: number of degrees of TEMPERATURE CHANGE required to change tire pressure by 1 PSI.
Temperature Difference Reference Table (Results in °F):
Cold-Tire Temperature (T1)
32 PSI
34 PSI
36 PSI
38 PSI
40 PSI
42 PSI
44 PSI
-40 °F
13.1°F
12.3°F
11.7°F
11.0°F
10.5°F
10.0°F
9.5°F
0 °F
14.4°F
13.5°F
12.8°F
12.1°F
11.5°F
10.9°F
10.4°F
32 °F
15.4°F
14.5°F
13.7°F
12.9°F
12.3°F
11.7°F
11.2°F
40 °F
15.6°F
14.7°F
13.9°F
13.1°F
12.5°F
11.9°F
11.4°F
50 °F
15.9°F
15.0°F
14.2°F
13.4°F
12.7°F
12.1°F
11.6°F
60 °F
16.2°F
15.3°F
14.4°F
13.7°F
13.0°F
12.4°F
11.8°F
70 °F
16.6°F
15.6°F
14.7°F
13.9°F
13.2°F
12.6°F
12.0°F
80 °F
16.9°F
15.9°F
15.0°F
14.2°F
13.5°F
12.8°F
12.3°F
This also illustrates the point I made above about the rule of thumb of "using 10°F for every 1 PSI". As you can see, that could lead someone astray for most driving circumstances we encounter.
In case you use the Celsius scale for your tires:
Temperature Difference Reference Table (Results in °C):
Cold-Tire Temperature (T1)
32 PSI
34 PSI
36 PSI
38 PSI
40 PSI
42 PSI
44 PSI
-40 °C
7.3°C
6.9°C
6.5°C
6.1°C
5.8°C
5.6°C
5.3°C
-10 °C
8.2°C
7.7°C
7.3°C
6.9°C
6.6°C
6.3°C
6.0°C
0 °C
8.5°C
8.0°C
7.6°C
7.2°C
6.8°C
6.5°C
6.2°C
5 °C
8.7°C
8.2°C
7.7°C
7.3°C
7.0°C
6.6°C
6.3°C
10 °C
8.8°C
8.3°C
7.9°C
7.5°C
7.1°C
6.7°C
6.4°C
15 °C
9.0°C
8.5°C
8.0°C
7.6°C
7.2°C
6.9°C
6.5°C
20 °C
9.2°C
8.6°C
8.1°C
7.7°C
7.3°C
7.0°C
6.7°C
25 °C
9.3°C
8.8°C
8.3°C
7.8°C
7.5°C
7.1°C
6.8°C
If you want to see how I computed the numbers in these tables, see Appendix B.
