Question Tire pressure - which number to follow?

snowbird25ca · Nov 2, 2009

I know we have some car gurus here, so I'm hoping someone has an answer and hopefully an explanation for me.

I checked the tire pressure on our new car tonight since the temperature has dropped below freezing, and discovered that they're inflated well above the amount stated on the manufacturer's placard inside the driver door. The placard says 30 psi for both front and back tires. The tire walls say to not exceed 44psi max pressure. The tires were all inflated to between 42.5 and 44.

So which is right? Obviously don't exceed the 44, but should I drop the pressure all the way down to 30psi? Or go more like 32 or so considering that the temperature is just going to get colder?

Or is it better to keep it closer to the max PSI stated on the tire wall?

The vehicle manual says to follow the placard inside the driver door. I don't understand why the dealership would've inflated the tires so high though.

We've only had the car around 6wks...

Judi · Nov 2, 2009

I would go by the tire, not the car.

BigDaddy · Nov 2, 2009

Short answer, the vehicle manufacturer spec is the correct one.

Long answer, here.

I normally run a little more than spec to get better gas mileage. The dealership adding 50% over spec is way off. That much will create a rougher ride, abnormal tire wear, and reduce your traction.

QuassEE · Nov 2, 2009

A little over also tends to reduce wear.

-Nicole.

snowbird25ca · Nov 2, 2009

Thanks Bigdaddy - and too funny that one of the best links I've come across actually happens to be on this forum. Everything I've googled has such a mix that I was starting to feel like maybe there isn't a right answer...

It didn't make sense to me though that our truck which carries more passengers and cargo, would have the same tire wall psi as the car, but the manufacturer psi was 5 higher than the car. The manufacturer # being higher for the truck made sense... but the max pressure on the sidewall of the tire didn't make much sense to me.

snowbird25ca · Nov 2, 2009

One other question - and this one makes me sound really dumb, but it happened almost instantly.

One of the back tires the cap got mis-threaded when I was putting it back on and now I can't get it off. When I let the little air out of the tires that I did let out, it went down to 40. I decided to let a little air out of all the tires, and ended up with the front one at 38.5. So I matched both front ones at 38.5 and the back ones are at 40 since I can't get the valve stem cover off of that one tire.

How much difference does that 1.5 psi between front and back tires make? And any tips for getting that cover off? My fingers were freezing by the time I came back to try again and I had troubles getting a good grip, lol.

BigDaddy · Nov 2, 2009

snowbird25ca said:
Thanks Bigdaddy - and too funny that one of the best links I've come across actually happens to be on this forum. Everything I've googled has such a mix that I was starting to feel like maybe there isn't a right answer...

Lol, there's no telling what kind of info you'll find on this forum.

snowbird25ca said:
It didn't make sense to me though that our truck which carries more passengers and cargo, would have the same tire wall psi as the car, but the manufacturer psi was 5 higher than the car. The manufacturer # being higher for the truck made sense... but the max pressure on the sidewall of the tire didn't make much sense to me.

Actually the difference in manufacturer specified PSI between different vehicles isn't as telling as you may think. The size of the tire plays a huge role. In order to support more weight (as in the difference between a car and truck) you have to increase the capacity of the tires. The two ways to do this is to either increase the pressure, or increase the size of the tire. You could see a 3500 lb car, and a 5000lb truck with exactly the same manufacturer spec simply because the truck has much larger tires.

BigDaddy · Nov 2, 2009

snowbird25ca said:
One other question - and this one makes me sound really dumb, but it happened almost instantly.

One of the back tires the cap got mis-threaded when I was putting it back on and now I can't get it off. When I let the little air out of the tires that I did let out, it went down to 40. I decided to let a little air out of all the tires, and ended up with the front one at 38.5. So I matched both front ones at 38.5 and the back ones are at 40 since I can't get the valve stem cover off of that one tire.

How much difference does that 1.5 psi between front and back tires make? And any tips for getting that cover off? My fingers were freezing by the time I came back to try again and I had troubles getting a good grip, lol.

The plastic threads inside the cap should be soft enough that they'll give way fairly easily, even if you cross threaded it when you put it back on (well, if it's cold there, the plastic will be stiffer and may make it a little harder). If you do have to damage it to remove it, make sure the threads of the valve stem are clear of any pieces of plastic so that you don't mess up the new cap.

A little pressure difference front to back isn't that big of a deal. Pressure differences left to right should be avoided, because they have a greater impact on handling (i.e. both front tires should match, and both back tires should match).

snowbird25ca · Nov 2, 2009

BigDaddy said:
A little pressure difference front to back isn't that big of a deal. Pressure differences left to right should be avoided, because they have a greater impact on handling (i.e. both front tires should match, and both back tires should match).

That part I knew, so I matched the left and right tires, just front and back aren't matched. It'll get above freezing tomorrow, so I'll give it another try to get it off.

It was the tire's sidewall max pressure that made me wonder actually how they could be the same. I guess the same principle applies with that though as with the manufacturer's rating.

beeman · Nov 3, 2009

For safety reasons I would prefer to run the pressure based on the vehicle placard. The tire is rated up to 44psi for its maximum weight rating, and also states to refer to the vehicle manual or placard for proper inflation. If the tire is not going to be used for the full weight rating, the tire will not need the full 44psi to perform properly, which is why your vehicle does not require as much pressure. At the full 44 psi, and half the load of weight rating you will have a rougher ride, poorer handling, and abnormal tire wear. As for front to back, my truck recommends 41 psi on the rear axle, and 35 psi on the front, so a variance in front to back is fine, however left to right need to be the same. Watch your tires closely as we head into the cold winter months.

snowbird25ca · Nov 3, 2009

Thanks Beeman and the others who have posted.

I was able to get the back cap off with the weather being a little warmer yesterday and let the air out of all the tires so they're now 3 psi above the manufacturer's placard. I figure with going in to cold weather it gives a little bit of breathing room and should still be within range until we get seriously cold. I'm sure as they are now they'll be fine down to -15 or so.

(Too bad my car's Onstar report doesn't send the tire pressure reading with the other info every month, it's nice having that info so easy to see for our Outlook.)

Question Tire pressure - which number to follow?

snowbird25ca

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Judi

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BigDaddy

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QuassEE

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snowbird25ca

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snowbird25ca

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BigDaddy

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BigDaddy

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snowbird25ca

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beeman

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snowbird25ca

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