# Checking tire pressure with tires off the vehicle vs on

#### bottgers

Yesterday while I rotated my tires, I decided to check the pressure while I had the tires off the vehicle. I inflated the tires to the usual 35 psi and installed them on the car. This morning I decided to check the pressure again (using my trusty digital Accutire gauge) and all 4 tires were at 33.5 or 34 psi. I'm going to rule out the possibility that all 4 tires leaked out essentially exactly the same amount of air as the odds of that occurring are astronomical. I'm thinking the difference is due to checking them while off the car vs on. However, if anything you'd think the pressure would read higher when the tires are on the vehicle because of the added weight of the vehicle. It doesn't make sense to me that it would be the other way around. Can anyone explain this?

#### DJ

Can I safely guess yesterday the tires were laying flat out fully exposed to the sun????

#### Kestas

Staff member
The sun would actualy affect the tire more like 2-4 psi. The variation you experienced was probably a drop in temperature. Check it again later in the day.

#### wantin150

I would say that there was a 15 degree negative change in temperature from when you checked them the first time. Tire pressure will change +/- 1 PSI for every 10 degrees in temp. In the winter, I set my tires at +3 over the mfg recommend pressure for the lowest expected temperature. So, at 20 degree outside temp, my tires will read 35 psi. Theoretically, they will never be under inflated due to temp changes. This is important if you adjust your tire pressure to mfg in a 50 degree garage and then drive outside in 0 degree temps. Your tires are already 5 psi low.

#### CapriRacer

Some time back, a similar question came, so I went into our vehicle shop and there was a Buick LeSare on the lift. I measured the front tires with a pressure gauage that could read to the nearest 0.1 psi. Then I lowered the car to the ground and measured the front tires again and got a gain of 0.5 psi. I didn't do the rears - the results would have been lower.

#### XS650

quote:
Originally posted by obbop: Hmmmm...... Conservatives: It's 10 degrees you liberal scum!!! Liberals: You greedy anti-people profit-driven punks.... it's 15 degrees!!!!! Moderates: Sheeesh...make it 12.5 degrees and get on with the oil change, why dontcha'? The Silent Majority: what's an air gauge?
Sounds like the 10/20/70 rule. 10% of the people make things happen, 20% watch what's happening and 70% wonder what's happening.

#### DJ

Temp drop is where I was heading. Another thing to consider is always try to use air out of a tank compressor. The compression of the air makes at least some of the moisture fallout to the bottom of the tank, a portable tire inflator type compressor will just send that moisture along into the tire.

#### bottgers

Using a tank compressor isn't an option for me. The only compressor I have is the portable one I keep in the trunk.

#### bottgers

It probably was the temp. Yesterday when I checked them the temp was in 70's. This AM it was in the 50's.

#### XS650

quote:
Originally posted by bottgers: It probably was the temp. Yesterday when I checked them the temp was in 70's. This AM it was in the 50's.
Sounds right. It's about 1 psi for every 15F on a tire in the low 30 psi range. For some reason the conventional wisdom is that it's 10F.

#### obbop

Hmmmm...... Conservatives: It's 10 degrees you liberal scum!!! Liberals: You greedy anti-people profit-driven punks.... it's 15 degrees!!!!! Moderates: Sheeesh...make it 12.5 degrees and get on with the oil change, why dontcha'? The Silent Majority: what's an air gauge?

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