I read the High Speed Driving post in Tech Help and thought I'd open it up here to get more input (and to see how quickly the folder lights up in flames )

My German cars each have a sticker on the fuel door that lists pressures for front and rear tires, for light and heavy loads. Yes, each car is different.

Also, they each note to add pressure for speeds over 100mph, again a different amount.

My first understanding is that these pressures are only valid for COLD tires, and that COLD means overnight and not just sitting for 30 minutes after driving.

How long DO tires take to assume "room temperature" anyway? Has anybody studied this?

Anyway, my second understanding is that the 100mph speed means SUSTAINED speed, like 30 minutes or so, and not a 5 minute sprint down the road before they catch you on radar.

As a quick side note, motorcycle tires are high performance and subject to wider ranges of load and speed than most cars. Articles I have read about proper pressure for motorcycle tires say that the BEST indicator of proper tire inflation is how much the pressure RISES from cold to normal operating temperature. I seem to remember a 10-15% rise is about right, and that this would then raise a 30 psi cold reading to 33-35 psi at normal temperature. A higher rise means that the cold pressure is too low, while a lower rise means it's too high.

I also understand that's how the real racers do it; set a pressure cold, run the car until the tires are HOT and check, then adjust the COLD setting.


I'm looking forward to reading a lot of information here soon, particularly variations by tire manufacturer, model, and car chassis!

I know who let the dogs out; I did!

BCingU, Jim

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'96 E300D 60k mi (wife's daily ride)
'95 Audi 90 120k mi
'92 GMC Suburban 139k mi
'85 300SD 234k mi (my daily ride)

i believe that one of the reasons for increasing the inflation pressure for sustained high-speed driving is to reduce the sidewall flex at those high speeds. Sidewall flex creates internal friction, which raises a tire's temperature.

the same is recommended if you go autocrossing, so you don't run your tires off your rims.

different cars have different suspension design characteristics, and the tire itself is part of the suspension. Inflation pressure determines the amount of road shock the tire absorbs, and the shape and behavior of the contact patch, among others.

i've read somewhere that it takes a few hours for a tire to return to normal temperature, since rubber is such a good insulator. Also, if you cannot inflate your tire to the recommended pressure before driving off, be sure to do it within about 1-2kms (and drive slowly) or else your tire will have heated up considerably and you will get readings that are way off. Even then, you should provide some margin.

also, invest in the best tire gauge you can get your hands on.

my Can$0.02