# I got new tires on my car, but the tire store over inflated them. Is there a reason? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [June 9, 2022, 3:29am UTC](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904 "2022-06-09T03:29:38Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [June 13, 2022, 3:15am UTC](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904/21 "2022-06-13T03:15:12Z")
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. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 13, 2022, 11:00am UTC](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904/22 "2022-06-13T11:00:55Z")
> [@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](https://boards.straightdope.com/t/car-tire-pressure-at-high-elevation/958063/19): > > 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… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 13, 2022, 11:34am UTC](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904/23 "2022-06-13T11:34:04Z")
Ah, good catch! Thank you. With that correction, it actually turns out to be uncannily precise for a rule of thumb. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 13, 2022, 6:11pm UTC](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904/24 "2022-06-13T18:11:53Z")
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. 🙂 [Previous page](https://boards.straightdope.com/t/i-got-new-tires-on-my-car-but-the-tire-store-over-inflated-them-is-there-a-reason/965904.md?page=1)