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I measured the PSI on my rear driver tire (using a gauge pin - push it into the needle and a stick pops out with a reading), and I got a reading of 22 PSI. My tires are supposed to be anywhere from 30-34 PSI. The thing is, the rear driver tire (the tire in question) doesn't look flat at all. It looks just as good, if not better than the other tires.

Should I fill the tire with air? I don't want to over-inflate the tire, if my gauge is giving me a bad reading. However, at the same time, the gauge seems to be working fine for the other tires, so one would be reasonable to assume that it's working well for the tire in question as well.

Another note to consider - my front driver side tire looks flatter than the rear driver side tire, but it's PSI is 32.

What should I do? Should I go with how the tire "looks" or what my $4 (from Advanced Auto) gauge says?

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I would follow the gauge. Not how it "looks" like. I find it very strange that the pressure what is what you expect in the other tires. Could it be something wrong with the valve in that tire causing a faulty read out in the pressure gauge? – rana Jan 6 at 21:07
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My tires are supposed to be (...) 30-34 PSI I got a reading of 22 PSI. -> Of course. (also, any air pump in any gas station will give you a precise reading when you inflate your tires, so you know when to stop exactly.) – njzk2 Jan 6 at 22:23
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@njzk2 perhaps it might be more accurate to say "[...] any well-maintained air pump in any well-run gas station [...]" – O.M.Y. Jan 7 at 8:29
    
@O.M.Y. you are right. I tend to see things from where I am, but that may not be valid for other places – njzk2 Jan 7 at 14:45
    
@rana If there was a problem with the valve in the suspect tire it would probably not consistently give him the same 22lb reading every time. Also, even if it is just a partially blocked air-stem then blowing a little air into the tire would probably clear the stem and then a re-read of the tire pressure would tell the story. – O.M.Y. Jan 7 at 15:36
up vote 16 down vote accepted

Yes, you should fill your tire. The fact the other tires all are nominally correct means you should trust your gauge.

Modern tires often have reinforced sidewalls that mask the appearance of low pressure. Waiting for a bad wear pattern is not a good plan, as then you'll have an unevenly worn set of tires.

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thanks for the advice guys. Definitely will get it filled now – ralston3 Jan 6 at 21:28
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I'd also point out there's a difference between running low at 29 when you're recommended at 30-34 and running low at 22. After a low enough pressure, you simply won't be able to move anymore. If you're one or two low, you probably should get it filled, but hey it's super cold and you're getting an oil change next week - probably be fine. But 22, I'd go to the pump at the next available opportunity. – corsiKa Jan 6 at 22:53
    
I'd like to also point out for inexperienced drivers that if your tire(s) get too low then trying to drive on them will cause the metal edges of the hub to press against the inside of the tire and chew it up, thereby permanently damaging the integrity and safety of the tire. – O.M.Y. Jan 7 at 15:41
    
@corsiKa: "After a low enough pressure, you simply won't be able to move anymore" -- what? I've seen pictures of cars that have pulled into a shop driving on nothing but the bare rim. Unless you've modified your car to drop the suspension radically, you should still be able to move even if your tires are completely flat. Obviously a bad idea, but still possible. – TMN Jan 7 at 17:50
    
I had that happen with two flat tires on a moderately powered car, it just wouldn't roll... but not on one flat tire. – kmarsh Jan 7 at 17:59

Because you have verified the gauge is working reasonably by trying the other tires, go with the gauge. Your owners manual will have the inflation pressures for your car. Just make sure that the recommended pressure in the manual is less than the maximum tire pressure imprinted on the sidewall of the tire. If it is, just fill it to the manuals recommended pressure.

Hope that helps!

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Generally speaking any time your tire pressure is more than 10% over or under the ratings I would definitely correct the pressure ASAP and check the tread wear too. In your case 10% is 3 or 4 pounds and you are seeing the tire is low by about 8 pounds (20+%) which is way too much discrepancy. Pump it up and keep a close eye on it for a week or so to be sure you don't have a slow leak. If it drops again go see your local mechanic / tire guy.

Also, @corsiKa makes an important point above ... the temperature when you measure is also a factor. Extreme cold will temporarily lower the tire pressure a couple of PSI and extreme heat will similarly raise it temporarily. Plus long drives can heat up the air in a tire from friction even on a cold day. Keep these issues in mind both when measuring and when adding air to tires.

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Your description "front driver looking flatter" suggests that the front driver and rear passenger tires could be carrying more load than the rear driver. If the front driver and rear passenger are on higher pavement than the other two they will carry more load and could appear "flatter". Is this a possibility?

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yea I just found out today from a car junkie friend of mine that the front tires will look flatter due to the fact that the front of the car is much heavier than the rear. – ralston3 Jan 7 at 4:45
    
Another point: the front wheels are also tilted when you turn the steering wheel. I.e. when you turn left, the left tire stands more on its outer edge and appears flatter. – sweber Jan 8 at 7:13

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