X2Infrared thermometers are terrible for this. You need a pyrometer for accurate readings. I've done this at the track with my friend Bob's SpeedSource RX-7. To do it properly, you actually need a few people/pyrometers. We sat at the paddock just after some long S's. He ran the car for 4-5 laps and pulled immediately into the paddock. We ran out with pyrometers and measured the inside, middle and outside of each tire and recorded the results.
Like I said, you really need multiple pyrometers because the other tires will be cooling while you're measuring the first tire.