Tire pressure questions

[RatDog]

About six months ago, I got 4 new tires from the tire guy near where I work. In October, my oil change guy said my tires were overinflated by 8 pounds so he let some air out. I might be crazy, but I think I have a vibration that has gotten increasingly worse since then. A couple weeks before Christmas I got 4 new tires for my wife's car from my tire guy. He mentioned that her tires had uneven wear likely due to under-inflation. For Christmas, I received a neat little digital tire pressure guage. I checked the pressure of my tires and all 4 were between 32-33 psi. I assumed this is good because the placard inside my drivers door says 32 psi. I then checked my wife's tires and all were between 40-41 psi. This does not check with her placard which also is 32 psi.

Is it correct to think that the new tire pressure should be the same as the placard states? The tire sizes are exactly the same as the original tires, but are a different brand. Is that placard only applicable to the original tires?

I am certain that the next time my wife gets her oil changed, they are going to want to take air out of tires. Should she let them? Is the oil change guy ruining our tires? I would think the tire guy knows more than the oil change guy.

What do you guys think?

 

[Tarrant64]

If I'm using the tires recommended for my car, I will use the information in the door of my card(mine being 30-32). I thought some tires though have the ratings on them, and they don't always match what the car says.

During the summer and winter though you have to have either a little bit more or less pressure. I forget which though.

 

[MBrown]

You are supposed to go by what the placard says. You probably just need a balance.

 

[BlahBlahYouToo]

I pump mine up 3-5 psi more for better FE.

 

[Mermaidman]

Your oil change guy measured the pressure when the tires were hot, so I'm wondering how he knew they were overinflated by exactly 8 PSI. The recommended pressures are for cold tires.

 

[boomerang]

Originally posted by: Mermaidman
Your oil change guy measured the pressure when the tires were hot, so I'm wondering how he knew they were overinflated by exactly 8 PSI. The recommended pressures are for cold tires.

Aaaand, tire gauges can vary considerably. I've seen pencil-type gauges show a difference of 10 lbs.

 

[nakedfrog]

Originally posted by: Tarrant64
If I'm using the tires recommended for my car, I will use the information in the door of my card(mine being 30-32). I thought some tires though have the ratings on them, and they don't always match what the car says.

During the summer and winter though you have to have either a little bit more or less pressure. I forget which though.

Tires will have the MAX PSI on them, but that doesn't mean you should use the maximum allowable rating. You use the same pressure in summer/winter, but due to temperature fluctuations between seasons, the pressure will change if you never do anything about it.

 

[Zenmervolt]

Originally posted by: boomerang

Originally posted by: Mermaidman
Your oil change guy measured the pressure when the tires were hot, so I'm wondering how he knew they were overinflated by exactly 8 PSI. The recommended pressures are for cold tires.

Aaaand, tire gauges can vary considerably. I've seen pencil-type gauges show a difference of 10 lbs.

I've never seen a variation like that with a quality gauge, not even the older "pencil type". The problems are always due to either cheap gauges with a bad seal or user error. Even then, it's nearly impossible to get a bad reading from either a digital or dial gauge.

To the OP's question; the inflation pressures listed on the car's tire pressure tag represent a specific set of compromises chosen by the manufacturer. The manufacturer is balancing things like ride comfort, tire life, vehicle load capacity, and handling feel when they make that recommendation. If you keep to the manufacturer's numbers, you will be fine, but there's nothing wrong with inflating the tires to a greater pressure if the tires can handle it and they aren't wearing unevenly.

ZV

 

[Throckmorton]

Make sure you are measuring temperature of your and your wife's tires when they are cold. The variation shouldn't be that great though AFAIK