There are three ways I have found to reliably measure tire temperatures. I use the first two, and you may use all three if you have a Tire-Pressure Monitoring System (TPMS) that also measures the tire's internal air temperature:
infrared thermometer;
manual (hand) measurement to verify my infrared thermometer is working correctly;
some TPMSes have the ability to measure the tire's internal air temperature, in addition to pressure.
The first two are used to estimate the air temperature inside the tire based on the assumption that the tire's internal air temperature will—after some time of riding—come up to being close to the external temperature of the tire's rubber. (Hint: it does after about 20 minutes of driving.) With the third method, that temperature is measured directly so there is no need to estimate it.
With an infrared thermometer, measurement is easy: follow the instructions (typically point and pull the trigger to start measurement) and read the result on the display panel.
With hand measurement, measuring precisely can be a bit trickier, and will vary from individual to individual. So I recommend:
practice with a known-working infrared thermometer to increase your certainty and accuracy; and
build your own "How it Feels" table like the one below. This can be approximated with a glass of water, a microwave oven, and a food thermometer.
The following is what I use, is fairly easy for me to remember, and works well for my hands:
How It Feels
°F
°C
Like tap water
70
21
A little cooler than body temperature
90
32
A little warmer than body temperature
100
38
Hot, but doesn't burn
110
43
Burns after 15 sec
115
46
Burns after 10 sec
125
52
Burns after 5 sec
130
54.5
Burns after 3 sec
135
57
Burns after 2 sec
140
60
Burns after 1 sec
145
63
Burns after 0.5 sec
155
68
Burns after 0.25 sec
165
74
Tip: Use the fire-fighter's technique—test the temperature using the back of your hand so you don't burn the palm of your hand.
This covers most of the tire temperature ranges I have encountered during warmer months. Your needs may vary if your riding climate is much different than Colorado's.
