Opinion on Tire Pressure / Clueless mechanics?

[LillFilly]

I've had my tires 5-6 years, have put about 100,000 miles on them, they're all weather Nokians, and I regularly keep them inflated. The tires themselves say 350 KG / 51 PSI. My front driver's tire seems to lose air faster than the others; I think this one previously had a hole that I had fixed. So I brought it to Wal Mart to check for a leak, see if the tires needed to be rotated. Afterwards, one mechanic told me that my tires were fine. Then the other mechanic told me that my tires were overinflated and that's why my tires were wearing so fast I never complained about that. He told me that even though the tires say max 51 psi that I should only ever inflate them to 35 psi, and that that's what it says on the inside of my door. I told him that those weren't the tires the vehicle came with, so I went with what the tires said (note that for all these years not one of a dozen mechanics has told me my tires are overinflated) He then told me that because they were overfilled my tires were wearing in a wavy pattern. Umm, the first guy told me they were fine and didn't need to be rotated. When I went to get my car the first mechanic told me that, in fact, my tires said 35 psi, and that the reason they were losing air was they were leaking around the rims because they were overfull. In any case, they said there was nothing to fix and I wasn't charged.

Well, I checked my tires and they DO INDEED say 51 PSI. Now I understand that in warm weather to lower the pressure, and that the pressure expands after the car's been run, so I always leave a few pounds leeway. I'm just wondering what kind of idiot these guys thought I was that I had misread my tires for 6 years.

Q&A on the internet seems split between never inflating any tire more than 40lbs, to just go by what the tire says. What do you guys think?

[Aethian]

I believe that the 51 on the tires are what they can be safely inflated to, but that you should go with what your car says. But I'm guessing...

100,000 miles on tires are excellent but when they continue to loose air like that it's time to think about pricing new tires. I had a tire that liked to leak on me and I checked it every so often and then when driving 70 on the highway in front of a semi...my tire blew. Thankfully I caused no problems and the semi driver was far enough behind to change lanes and I could get off the road safe enough.

[teh_blumchenkinder]

The front driver's tire thing seems to be a normal thing-- mine does it too, and my dad never really comments on it being odd, and he's worked with cars most of his life (from about 10 to 45), doing most of the family's car maintenance and upkeep. (His dad/my Opa taught him, and he owned a Napa parts and garage. )
35 to 51 PSI is kinda huge. Go with tires. Manufacturers know their own products (for the most part), and please realize I only know enough to be dangerous. (ie I am so not a mechanic it's not even funny)

[Chromatix]

The correct tyre pressure depends on three things:

- The width of the tyre. This probably doesn't change much between the car's original tyres and the Nokians. Usually it is determined by the wheel rim.

- The weight of the car. Ths is why front and rear tyres often get different pressures, and often you should have a different pressure for rear tyres if you have a full load of luggage. At the correct pressure, the bottom of the tyre will be maximally flat and gripping the road.

- Adjustments for unusual conditions. For example, less pressure in hot weather (because the heat from driving will increase the pressure further), or a temporary partial deflation to get yourself out of a poor traction situation (because the ground isn't flat and you need to let the tyre conform to the ground better).

In summary, you should probably be using the pressure specified by the car manual, rather than the pressure specified on the tyres. That latter number is what the tyres are structurally safe at, but might not be ideal for your particular vehicle.

Bicycles are a different matter - all bicycles weigh about the same (although their riders don't), so tyres have the recommended pressure on them rather than the limit pressure.

[Aethian]

Hey I was close, thanks Chromatix.

[protege]

After 100,000 miles, I think you've gotten your money's worth out of them. I'd probably replace them. Tires also suffer from dry rot--the rubber degrade over time. Sure, the tread might be OK, but the sidewalls could be bulged or cracked--weaknesses that might let go under heavy cornering.

The "51" on the tire is probably the *maximum* tire pressure. Some people lay their cars up over the winter, and will increase tire pressure to prevent flat spots. Actual safe pressure for driving is well below 51--most cars I've had usually say 35-40psi.

BTW, I've never heard of a tire leaking from "too much" air. Usually, over-inflation simply causes the center of the tread to wear. If a tire is leaking, it could be a failing valve stem, a stone (or nail) in the tread, or even from faulty materials. Valve stems are cheap to replace, and most holes in tread can be filled.

[pitmonkey]

If you look at the door jam of the car (depending on the car could also be in glove compartment or gas fill door) It will say what the psi should be at. A few years ago the psi was wrong on some ford SUVs and caused blow outs.
The problem with over inflation is that the car does not handle well in extreme conditions, Hydroplaning and on ice are not good. Also you are more likely to have a blow out if you hit something.

[Gravekeeper]

Oddly, Mythbusters did an episode on this. The conclusion was to leave them at the PSI recommended by the car/tire manufacturer. Otherwise you ended up with too much wear/worse fuel mileage, or poorer vehicle control.

Recommended for a reason me thinks.

[Andara Bledin]

Yup, go with what the car says for pressure.

Tire wear from underinflation will cause extra wear on the outside edges of the tire. Tire wear from overinflation will cause extra wear on the center of the tire.

Other forms of uneven tire wear are more likely related to alignment and/or suspension issues.

^-.-^

[It's me]

Mostly repeating what others have said but........

-Pressure stamped on the tire is the maximum pressure the tire is designed for
-The correct pressure to use for the tires on your particular car is on a sticker on the driver's door jamb
-If you use non-stock-size tires you may want to tweak the pressure a bit to ensure even tire wear... +/- a couple of pounds at most
-Both over inflation and under inflation has a negative affect on your car's handling and stopping.
-Tires deteriorate even if they aren't used. The rubber hardens over time, and the UV from the sun accelerates the process
-Five years is quite old for a set of tires, even if they had been sitting on a shelf (note: tires have manufacture date code stamped on them!)

Edit: More info on tire expiration dates:
http://www.aa1car.com/library/tire_expire.htm

And tire pressure:
http://www.aa1car.com/library/tirepres.htm

[Seshat]

Yup, go with what the car says for pressure.

Tire wear from underinflation will cause extra wear on the outside edges of the tire. Tire wear from overinflation will cause extra wear on the center of the tire.

Other forms of uneven tire wear are more likely related to alignment and/or suspension issues.

I use what the car says as a starting point.

I keep an accurate tire tread gauge. Between once a week and once a month, I check the tread on the outside, middle, and inside of the tires, for inflation. See above for how I fix it.

If the inside is wearing most, middle somewhat, and outside least, the tire is misaligned. Ditto for the other way around.
Wear in a wavy sort of pattern is axle or alignment trouble.


Correct wear is even all the way across, and even all over the tire (no flat spots, no bumpy spots.

[Der Cute]

100K miles - ready to ditch.
Usually tires are 40K, 60K, 80K.

I'd really start looking, NOW. I know someone who had the tires on the car for 14 years. Even though the mileage was low...they were just old.

Good luck!

[Docmayhem]

...but what are you driving?

I'm going to buck the trend here a little bit.

I'm also going to guess that you drive a pickup or full-size van/ SUV.

If the tires are passenger type tires, on a less than 4000lb (1900kg or so) car then you probably should not be running them at 51psi.

These sound more like light-truck (usually marked "LT") tires, which are made to handle higher loads than passenger tires, and typically should be inflated to 50psi.

As a lark, you may want to look at the tire pressure rating on the next semi you see in a parking lot - those often run 100 to 125psi.

[LillFilly]

I'm going to buck the trend here a little bit.

I'm also going to guess that you drive a pickup or full-size van/ SUV.

If the tires are passenger type tires, on a less than 4000lb (1900kg or so) car then you probably should not be running them at 51psi.

These sound more like light-truck (usually marked "LT") tires, which are made to handle higher loads than passenger tires, and typically should be inflated to 50psi.

As a lark, you may want to look at the tire pressure rating on the next semi you see in a parking lot - those often run 100 to 125psi.

Sorry. I have a 97 Toyota RAV4; a lightweight not-quite-SUV called a 'crossover utility vehicle.' It seems stock tires call for something in the neighborhood of 30psi, which seems awful low to me, but then again, my car is hardly ever heavily weighted down. I just can't believe this has never been told to me before.

[kpzra]

I'm going to buck the trend here a little bit.

I'm also going to guess that you drive a pickup or full-size van/ SUV.

If the tires are passenger type tires, on a less than 4000lb (1900kg or so) car then you probably should not be running them at 51psi.

These sound more like light-truck (usually marked "LT") tires, which are made to handle higher loads than passenger tires, and typically should be inflated to 50psi.

As a lark, you may want to look at the tire pressure rating on the next semi you see in a parking lot - those often run 100 to 125psi.

200 psi on my fuel truck at work. So glad I wasn't working the day the bead went on one of the rears.