Tire pressure variations

Hi guys!

I have a 2017 Mazda CX-5 with 19 inch rims. I stopped at the gas station to check my tire pressures and top them off. My vehicle calls for 35PSI front/back. Anyway, I set the air compressor at the gas station to 35 and topped them off.

Upon getting home, I checked them with my digital gauge and they’re showing 37PSI when cold. I lowered them to 35, but was wondering if 2psi extra would actually make a difference? The tires have 8k miles on them.


No it will not. Seriously , you really need to address this worrying you have about little things.


Just curious…

No problem. I put in 2 psi extra in mine as a matter of course.

If you are lucky, the gages are accurate to ± 1 PSI each. that means you can have a 2 PSI difference worse case. Plus differences caused by temperature change.

Did you let the tires cool off for an hour before taking the second reading? that can cause errors also. The tires would be warm just after driving.

Most people recommend setting the tires 2-3 PSI above the recommended value for just these reasons.

The problem there is, you don’t know how accurate the air pressure regulator for the compressor at the gas station is.

That’s why you check the tire pressure with a known accurate tire pressure gauge after adding air to the tires.


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What over PSI would it take to change the contact patch on the tire?

Gas station compressors tend not to read accurately. It would be best to check them at home (in the morning) with a good dial-type analog gauge.

2 PSI now isn’t a big deal, but temperatures are rising, so that could easily be 5 PSI high before too long.

The size of the contact patch is directly related to inflation pressure - meaning that even 0.00001 psi difference will change its size.

But I think you want to know how much pressure would be a significant enough change to worry about - and as others have said, if you are within a couple of psi, it doesn’t matter much. If you are looking for a practical solution, keep in mind that UNDER inflation is much, much worse than overinflation. Try 2 to 5 psi ABOVE what the vehicle tire placard says - and do this on a warm day. That way you’ll always have enough pressure in the tires.

And - No! - That amount of over inflation will not cause wear issues by itself. There are other things that have a much greater affect - alignment being the one most likely to cause problems.


Volvo, I hope that was meant as a joke, because if it wasn’t, you are very rude.
Come to think of it, since I can’t tell if it was a joke, it is rude!

In cases like this you have the right to remain silent… please consider it.
CSA :palm_tree::sunglasses::palm_tree:


Just 1lb will change the contact patch, but very difficult to measure accurately.

And unless your digital gage has been calibrated and a compensation curve created, you really don’t know which is correct. 2PSI is within the acceptable error tolerance of the average pressure gage. Fortunately, +/- 2PSI is meaningless in the average car.

First off, those gauges are cheap. Keep one in your glove box.

The 19 inch rims are really low profile tires. I’m surprised at the factory setting is that high on a low profile Tire but you have the manual so you know.

2 pounds of air higher on a low profile Tire would be noticeable in ride quality. Those kinds of tires have the tendency to make the car ride like a Buckboard anyway. Two extra pounds can also make a noticeable difference in mileage on long trips by making the tire harder and making it roll easier, but harder means less grip, especially in the rain or snow hello oh yeah. Or around town driving if you don’t mind a little extra tire wear I recommend two pounds less than but you have to check your air pressure more regularly as it gives you less cushion just for the natural leakage that happens over time. But the tire wheel grip better in the rain and the snow with slightly less tire pressure. You see that in auto racing all the time with teams adjusting the way the car handles by adjusting air pressure. In a race cars or drag racing cars and motorcycles, fractions of a pound make a difference.

In my old Chevy Prizm I can tell the difference in ride with a 2 pound difference.

Seriously :face_with_raised_eyebrow:

My two cars with low profile (50 series) tires have 33 and 35 psi recommended pressures, so 35 is nothing unusual.

Most people can’t tell 2 psi difference. I can as well, however.

yea, and if you have to make a hard stop in the rain your car will know the difference.

Yes. 32 pounds summer, 30 winter. If i notice it riding too nice I check the tires. Pump them back to 32 and bad city roads are thumping.

Which reminds me, my left front is probably down 2 pounds right now. Slight left pull and I can tell the difference in the way it squats sitting compared to the right.

I’ve heard of crank calls… No offense, but In addition to checking tires for a 2 pound differential, you do occasionally hop on the bathroom scale, right Mr. Crank?
CSA :palm_tree: :sunglasses::palm_tree:

That’s a good question for a physics exam. My guess is that contact area is proportional to the vehicle weight divided by the tire pressure. So you’d get a decrease in contact area by a factor of 35/37 comparing 37 psi to 35 psi. About a 5% decrease.