Air pressure variances

[talnlnky]

So... took the yaris in to redeem one of my free oil changes (not too much a fan of castrol syntec which I tried the past 5k). Had them rotate the tires (was free) and they commented that my tires were at 49psi. Now... I know its easy to get messed up readings on guages easily. But last time I touched the tires they showed 44ish +/- 1psi. I cited that I ran them around 44, and that it must be the warm weather (bout 30-40F higher) and the dude said that'd only make 1-2psi Difference.

Is he right? does ambient temp barely make a difference?
For the record... i'll be pumping my tires back up to 45 after a few days of sitting at 40psi (which they dropped it down to). They tried to convince me to put them back at 32, but I think they & I didn't want to argue, and I sure don't want them noting that I'm going above spec in their computers, so i didn't argue.

Noise does seem to be lower at 40 which I do like. Makes me wonder how quiet my car would be if I dropped back down to 32 like I ran for the first year of its life.

[CTScott]

If memory serves me right, the pressure will change by 1 PSI for every 10 F change in temp.

[IllusionX]

Do not forget that when you drive it, the tires to get hot(ter)

I remember when i first got my yaris (ok, 6 months later), i brought it back, after being stored for the winter (OUTSIDE). I had not touched the tire pressure since. They probably also checked the tire pressure while they did the oil change. I had not really paid attention to it, but bout a week later, i found that the tires seemed a little hard, as each bump is really being felt inside.

I know my dealer puts them at 32psi, it is a good old habit of the mechanics over there.

Finally, i decided to check my tire pressure, on a hot summer day, after a 50km ride on the highway and they showed 44psi.

Recently, i've been trying to adjust them to 33psi all around, and i fail miserably lol. I usually do not have time to check them first thing in the morning before school. Last thing i remember, i set them to 33psi, on a non sunny day. One morning, i got to check them (30psi), so i took my electric air pump and drove off to school. After school, sun sets on one side of my car. That is when i checked my tire pressure again and it showed 30psi on one side and 35psi on the other!!

[Cosmonaut]

also remember to use a quality gauge. the pen style gauges are less accurate than the dial gauges. some people have reported up to a ~7psi difference.

[R2D2]

I agree with CTScott- a tire will gain approximately 1 psi for every 10 deg F increase in temp and lose 1 psi every 10 deg decrease in temp. Tire pressures should be checked cold, which according to most owners manuals I have seen means driven less than one mile in the previous 3 hrs. As noted above, they build heat when driven on.

R2

[2009Toyotoad]

Originally Posted by CTScott

If memory serves me right, the pressure will change by 1 PSI for every 10 F change in temp.

That's on the conversion chart that came with my tire gauge says. However, my Dual foot truck tire gauge has a bigger pressure difference (range + as much as 7 psi) as you go down and across the chart. I assume to account for tire size.

[Loren]

Tire pressure changes due to temperature are relative to the beginning tire pressure.

For instance, if you start at 28 psi, and the temp goes up 20 degrees, you might see 30 psi. If you start at 44 psi and the temp goes up 20 degrees, you might see 47-48 psi or more. "1 psi per degree of temperature" is only going to be accurate (relatively) for a particular range of pressures, probably somewhere around 25-30 psi. It might be better to look at it as a percentage change. Say... 10% for every 30 degrees or something like that. Then, 30 psi would become 33 with 30 degrees of temp increase, and 44 would become 48.4 with the same temp increase. A little more realistic.

Best bet, especially when running really high pressures, is to check your pressures with the tires HOT because there's going to be a HUGE variance in pressure between cold and hot. Typical tire tread temps are up around 110-130 degrees after you've been driving for a while. If you started at 80 degrees, that can be up to 50 degrees of temperature change... enough to make 45 psi cold equal well over 50.