5

A few months ago, my tire pressure dashboard light came on after I bumped a curb a bit with my 2008 Toyota Yaris's rear right tire. I drove to a gas station and tried to pump up my tires, but the gauge was broken, so I left.

Later, I got a Black & Decker cordless tire inflator and gauged each of my tires:

  • Front left: 70 psi
  • Front right: 90 psi
  • Rear left: 20 psi
  • Rear right: 20 psi

The door well label says that all four tires should be at 32 psi cold inflate. I inflated my rear tires up to 32 psi. I read that overinflated tires can be dangerous, and here's where my confusion sets in.

My Black & Decker Cordless AirStation gauges the front left tire at 70 psi, above my door well's recommendation of 32 psi. But when I use a flathead screwdriver to slowly deflate the tire down to 60 psi, it starts to look saggy.

I thought this could be an artifact of deflating the tire in a fixed place, so I packed up, pulled my car out a bit, and rolled back into the parking spot. The tire still looked saggy, so I reinflated it back up to 65 psi.

I need an outside opinion on this, because I'm getting conflicting information:

  • Door well says all my tires should be inflated near 32 psi
  • Various Internet forums say my tires should be inflated near 35 psi front / 33 psi rear
  • Black & Decker gauge says my front tires are overinflated (70 psi and 90 psi)
  • Black & Decker was at low battery after 10 minutes fresh out of the box, refused to pump anymore
  • But Black & Decker gave consistent gauges for each individual tire
  • Deflating them slightly (60-65 psi) makes them look saggy, underinflated

Is my gauge functioning correctly? Does my door well recommended inflation label have a misprint? What should I do?

5

Reality check #1:

A tire at 70 psi would be bulging at the seams - you would be scared of the tire's appearance.

Reality check #2:

The sag on a tire at 20 psi is easy to notice.


I believe what's happening here is that the B&D inflator is low on charge and/or in the wrong setting, causing erroneous readouts for tire pressure. This isn't far-fetched given that the device has a digital readout.

As a sanity check, I'd recommend buying a cheap, simple, mechanical tire pressure gauge to verify the values. Regardless of brand, the gauge should be dependable and tell you what is really going on.

Tire pressure gauge

  • Yes, I failed to obey the B&D inflator's setup instructions of charging for several hours before use. Hopefully this will clear things up. Will update soon. – mcandre Feb 5 '15 at 4:08
3

Modern radial automotive tires often look saggy at proper inflation pressure, so I suspect you are over inflating the front tires to get them to look right.

I also suggest getting a second gauge, but would caution that the pop-out stick type pictured above are notoriously inaccurate and would suggest a mechanical dial gauge as the best type to buy. With the price of tires, this is well worth a second opinion!

I would measure the tire cold and use the cold inflation pressure listed on the door. This is what the car was designed for. Alternatively, a little more pressure (e.g. +5 psi) would make the car slightly more fuel efficient with slightly worse ride quality, but, within reason, is not as much of a problem as running too low. Of course 70psi-90psi is crazy high.

The rise in pressure with temperature when driving is not a problem. If you fill your tires on a freezing cold day (273K) and drive them up to 100F (312K) Ideal Gas Law says you will go from 32psi to 37psi, which is an entirely reasonable pressure to run your tires at.

2

It's not a good idea to measure the pressure when they are cold because because they will change pressure (potentially a lot) when they warm up, especially if you live in a cold part of the world. And the warm pressure is what matters, because that's what they will be while you're actually driving.

Measure the pressure after driving normally. For example when you get home from work or when you stop for gas.

Anywhere between 28 and 38 while warm is good, depending on how heavy the car is and how much weight you have loaded (four adults and luggage in the car, you want them harder).

In general, soft tyres bulge more and have more grip, making it safer to run lower pressures. If they bulge too much, they will get too hot and you will damage the tyres.

In very hot weather, maybe run them a bit harder. In cold weather a bit softer.

I personally use 34 on the back and 36 in the front on my car. The yaris is a smaller car than mine, so I would probably do 32 rear and 34 front.

70 or 90 psi is insane, and dangerous. Who made them that hard? Are you sure your equipment is not faulty? I would measure them again, perhaps at a gas station. And then drop the pressure to 34 (warm).

  • I would have to disagree with you about when to check tire pressure. It's the cold pressure you should be worried about, not the warm pressure. Check links on this Google Search. Tire Kingdom clearly states, "Tire pressures are always given for COLD tires. So check the tires in the morning or after they have been sitting for a couple of hours." – Pᴀᴜʟsᴛᴇʀ2 Feb 5 '15 at 17:37
  • @Paulster2 if you are going to just set your tyres to whatever the manufacturer recommends, then yes you should set them while cold because that's what the book says. But it is the pressure while warm that actually matters and you should be using that if you want optimum performance. The manufacturer's recommended pressure is never the best pressure, because they don't know what temperatures you're driving in, how fast you're going, how much weight you have loaded into the back, what the road surface is or what tyres you have fitted to the car. – Abhi Beckert Feb 9 '15 at 22:04
  • Actually, tire manufacturers have taken these factors into account. Really, what you are suggesting is completely wrong. In actuality, I think you've made my point with your comment. The fact the tire manufacturer does not know your temperature conditions you speak of is precisely why you should check them cold as they suggest. How are you going to know what the optimal pressure is when warm? Do you have a computer which you can plug all the parameters into and come out with the proper pressure? Then, are you going to get out and adjust the pressure every time one of them changes? – Pᴀᴜʟsᴛᴇʀ2 Feb 9 '15 at 22:30
  • @Paulster2 yes I am the sort of person who does use a computer to figure out ideal tyre pressures (mclarenelectronics.com/Products/Product/ATLAS) and yes I do get out and adjust my pressure when one of the parameters changes. It's not necessary of course, but I do it - mostly because it's fun. shrug – Abhi Beckert Feb 9 '15 at 22:54
  • As for how you know the optimal pressure, by braking hard until the wheel skids (or ABS activates), then change the pressure and do it again. Personally I do tests like that in various road conditions whenever I change the type of tyre I'm running. – Abhi Beckert Feb 9 '15 at 22:58
0

For somebody looking for the information about which tire pressure is right for this particular car model, my answer to this question: Tire-pressure on Toyota Vitz 2012 ...might be useful.

Short summary:

  • 2.2 bar front and rear for normal conditions (this is 32 PSI, so yes, the information you have is correct)
  • 2.4 bar front and rear for high speed driving

This is for 185/60R15 tire size, so be sure to check you have the same size!

Slight overinflation is not very harmful, so if driving at very high speeds only occasionally (of course, only if it's legal where you live), you can inflate them to 2.4 bar. It won't hurt even when driving at low speeds.

-2

you should look on the sidewall of the tire, it will have the suggested tire presser there. kinda hard to find sometimes, it will be there, just look real good....

  • 4
    The sidewall will have the maximum tyre pressure marked on it, not the recommended one, as that can be different for every model of car (and even for the expected load in the car) – Nick C Aug 31 '18 at 10:41
  • The sidewall pressure figure is the maximum the tire should contain. This figure is not the recommended pressure, it is the "never exceed" pressure. – DavidSupportsMonica Sep 7 '18 at 3:54

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