I got new tires on my car, but the tire store over inflated them. Is there a reason?

[pulykamell]

Well, I can’t argue math and physics. The car reports ranges from 35-39/40 psi and it’s been 60 - mid 80s here. I’m sure there’s plenty of rounding involved and I don’t know how accurate these sensors on the car are.

[Machine_Elf]

@Chronos

Shouldn’t be quite that much, but in that ballpark. Ambient temperatures will be around 520-530 F above absolute zero, so a change of 10 F is a little less than 2%. A 2% increase from a base pressure of 35 PSI would be 0.7 PSI.

Presumably you’re talking about 35 psi-gauge, but for this math you need absolute pressure: 49.7 psi. 2% of that is 0.994 psi.

Don’t feel bad. I made the same mistake just a few months ago:

Car tire pressure at high elevation

Actually it turns out I’m wrong because simple math is hard. Ideal gas law says that for a body of ideal gas at constant volume, the pressure and temperature are proportional. This means that you can calculate pressure and temperature changes like this: P1 / P2 = T1 / T2, or P2 = P1 * T2 / T1 The trick is that all of those values have to be expressed in absolute. So for a tire starting out at 35 psi-gauge, P1 = 35 + 14.7 = 49.7 psi-absolute. And if it starts out at 75°F, then T1 = 75 + 45…

[Chronos]

Ah, good catch! Thank you. With that correction, it actually turns out to be uncannily precise for a rule of thumb.

[ftg]

What with so many tire dealers charging a surprsing amount for the very precious and useful nitrogen for inflating tires, you’d think they wouldn’t want to waste such a valuable resource.