**Q**: *“I’m looking for the formula to adjust my cold tire pressure based on the ambient air temp. I had heard someone at the track mentioning this but missed getting the formula. I have used the pyrometer to ensure I have a tire pressure that gives a very even temp across the tire, let the tire cool overnight and took the tire pressure as my future starting cold tire pressure. At the same time, I recorded the ambient air temp to have record of both. My question is, if on the next time to that track, the ambient air temp is higher or lower than the day I recorded my previous tire pressure, how much do I adjust my cold starting pressure? Of course, I have done all of the above for each tire, arriving at four different starting cold pressures. Is there a formula that will give me the adjustment?”*

**A**: I’ve spent many years around some of the best race engineers and tire gurus, but I’ve never heard any of them talk about a formula to adjust cold pressures based on ambient temperature. I’ve heard and seen them make an educated estimate of reducing or increasing the pressures based on the current temp – but never seen one use math or a calculator to do so. They know that, for example, the pressures typically rise 4 or 5 or 6 or whatever amount. When the tires are cold, they start them that amount below the target hot pressure.

The way you’re going about finding the ideal hot temp is good. The key is finding the ideal hot temp – the temp that results in the best grip levels. Do you know how many psi the tires usually rise from the cold pressures? If you know they usually rise 4 psi, or 6 psi, or whatever, subtract that from the ideal hot pressures, and that’s your starting pressure. If the ambient temperature is colder than usual, start a couple psi higher; if it’s hotter than usual, start a couple lower. In a short period of time you’ll be making educated estimates, but know that it’ll never be perfect. Why? Because different tracks have different surfaces, which will also affect the amount the pressures rise. Also – and this may be the biggest difference – how hard you’re driving your car will have an impact on this.

There may be a formula for this, but I’ve never heard of it.

Hi Ross,

Michelin Tire Engineer in IMSA here, so this is my field for nerding out.

We do have a “rule of thumb” for adjusting tires based on ambient temperature, but track temperature also plays a huge role into our tire pressure setting.

For our IMSA slicks the ambient rule of thumb is +/- 0.6 psi for every 10F of temperature change, but the direction is the opposite as stated in your answer.

For example: let’s assume track temp is the same for both sessions, and a driver sets his tires at 70F on Day 1 and they hit his hot target on track. The next morning he comes to the track and temps are 50F…he will want to bleed 1.2 psi from the Day 1 starting pressure to hit the same hot target. This is due to the ideal gas law…you’re getting a larger delta of temperature growth (from 50F to the hot tire temp vs 70F to hot) and therefore experience a larger pressure growth.

Hope this makes sense, and happy to talk tires anytime.

Thanks for all your podcasts and content and keep up the good work!

A rule of thumb is for each 10deg F temp rise a tire will gain .7psi. This is given in some Michelin presentations and can also be derived from the ideal gas equation (PV=nRT). So if you have data you are confident in and ambient is 15 degF higher, then lower the pressure an additional 1 psi, if cooler by 15 deg add 1 psi. Lots of assumptions in the rule of thumb but will give you an approximate adjustment to start with.