# Auto Tire Pressures **URL:** \ **Category:** Factual Questions\ **Created:** [November 15, 2007, 9:38pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596 "2007-11-15T21:38:22Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![Sean\_Factotum](https://avatars.discourse-cdn.com/v4/letter/s/50afbb/32.png) [@Sean\_Factotum](https://boards.straightdope.com/u/Sean_Factotum)\ **Post date:** [November 16, 2007, 5:30pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/21 "2007-11-16T17:30:26Z")
[QUOTE=Duckster] Ah, go get a quality tire pressure gauge. Never trust pressure gauges at gas stations or connected to air pumps. Then either adjust the pressure at home with your own pump. Or measure just before you go to the gas station for air, and once there, measure again before and while filling with air. [/QUOTE] As Hank Hill says, “Always have _two_ calibrated tire pressure gauges with you. Tire pressure is not something you want to monkey around with.” [QUOTE=Una Persson] Folks (in general) should just worry most about keeping the tires at the recommended pressures, not so much about trying to super-optimize things. [/QUOTE] So do you use the pressure recommended by the tire manufacturer, or the pressure on the placard inside the door of your vehicle? Because they don’t always match. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [November 16, 2007, 6:09pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/22 "2007-11-16T18:09:23Z")
[QUOTE=Sean Factotum] So do you use the pressure recommended by the tire manufacturer, or the pressure on the placard inside the door of your vehicle? Because they don’t always match. [/QUOTE] Typically, tires have a “maximum” pressure only on their sidewall, whereas the car will have a recommended pressure. And you should not ever go over the maximum pressure on the tire, of course. However, you get into sort of a grey area when you replace tires. Lets say that a stock car with stock tires has a maximum pressure of 35 psi, and a recommended pressure of 30 psi. Now, lets say the car owner upgrades the tires to some z-rated ones with a maximum pressure of 45 psi. at first glance, inflating those to 30 psi as per the car tag indicates might be good, as it’s below the maximum on the tire. However, you have to assume, I think, that the real thing the car would “prefer” is a recommended tire pressure that is about 86% (30/35) of the maximum. So now one is left with the question, “do I inflate the tires to 38.6 psi, or something else? Does the relationship work linearly? Are there other factors that I need to be aware of here?” Mercifully, tires have a substantial Factor of Safety on them, and you could be off by a bit and not suffer much for it, in reality. In the example above, I would likely round up to 40 psi, assuming the higher performance tires will work better at an operating pressure closer to their maximum, but I don’t have a handy cite to support that, that’s my educated guess. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [November 16, 2007, 6:10pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/23 "2007-11-16T18:10:13Z")
[QUOTE=Sean Factotum] So do you use the pressure recommended by the tire manufacturer, or the pressure on the placard inside the door of your vehicle? Because they don’t always match. [/QUOTE] There is no pressure recommended by the tire manufacturer. Tire manufacturers specify a MAXIMUM pressure which is not to be exceeded. Auto manufacturers determine the RECOMMENDED pressure for the vehicle/tire combination. ---
**Author:** ![Rhubarb](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@Rhubarb](https://boards.straightdope.com/u/Rhubarb)\ **Post date:** [November 17, 2007, 6:56am UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/24 "2007-11-17T06:56:49Z")
[QUOTE=Una Persson] That’s a difficult question to answer due to the variables involved. And do you mean under/over inflated with respect to the recommended pressure, or the maximum on the tire sidewall? If it’s with respect to the recommended pressure, then I’d say that slightly over inflating will be better off overall. If it’s with respect to the maximum allowable pressure, then err on the side of under inflating, obviously. Some people under inflate their tires slightly in the belief that it will give a greater contact patch and help with cornering, braking, etc. The truth is, it depends on the tire, the car, the road, and the surface. In some cases under inflating can give you poorer handling due to the geometry change of the tire, and even poorer braking for the same reason. Folks (in general) should just worry most about keeping the tires at the recommended pressures, not so much about trying to super-optimize things. The problem is not the diligent people who are +/- 2 psi from their optimal setting, it’s the **enormous** number of people out there driving on 10 psi with tires intended for 35 psi. When I worked at the auto shop back in the 80’s, one of my many gofer jobs (since I was the more junior mechanic) was to check and air up customers tires. And it was rare - very rare - that more than one tire on a car was within 5 psi of the recommended pressure. The number of times I saw tires at 10 psi or less was appalling. On one car I saw less than 5 psi on several tires, and I wondered how on earth the bead could still have a seal with pressure that low. Then again, given how many people had their cars towed in because they ran it dry of oil…I guess it’s no surprise. [/QUOTE] Thanks for the reply. I was definitely referring to the recommended pressure, of course. My reason for asking had to do with a.) the OP’s concern for the difference in pressure between cold/warm tires which I assume is going to be less than 2 psi and b.) my dad’s advice when I was a young’un to go slightly over the recommended because tires leak a little and slightly overinflating will give you a longer runtime between re-inflating the tires. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [November 17, 2007, 7:12am UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/25 "2007-11-17T07:12:42Z")
[QUOTE=Rhubarb] …and slightly overinflating will give you a longer runtime between re-inflating the tires. [/QUOTE] Not if you want to slightly over-inflate them the next time as well, it won’t. ---
**Author:** ![jillscott8763](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@jillscott8763](https://boards.straightdope.com/u/jillscott8763)\ **Post date:** [November 17, 2007, 9:46am UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/26 "2007-11-17T09:46:03Z")
[QUOTE=Zambini57] You don’t need an air compressor to top off your tires. A cheap bicycle pump works just as well, and doesn’t take as long as you might think. [/QUOTE] and driving to the gas station takes less time than you may think… ---
**Author:** ![jillscott8763](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@jillscott8763](https://boards.straightdope.com/u/jillscott8763)\ **Post date:** [November 17, 2007, 9:50am UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/27 "2007-11-17T09:50:01Z")
[QUOTE=A.R. Cane] You can also buy a small compressor that plugs into your dash. I had to stop at a gas station for air recently, first time in years. It was 75 cents, so I decided to check all four tires, just to get my money’s worth. The thing shut off before I could get to all 4. I’d guess it ran for 3 to 4 minutes. [/QUOTE] My guess is you got ripped-off. Probably less gas stations charge for air than you use the pump. ---
**Author:** ![Enright3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/enright3/32/2897_2.png) [@Enright3](https://boards.straightdope.com/u/Enright3)\ **Post date:** [November 17, 2007, 3:04pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/28 "2007-11-17T15:04:42Z")
[QUOTE=x-ray vision] What’s the difference between the pressure in your driveway and at the station? [/QUOTE] Am I the only one (besides x-ray vision himself I presume) that thinks a lot off this discussion wouldn’t be necessary if this question were answered? 😕 It’s the smartest reply, and it seem totally ignored. If the OPer check his tire pressure at home, and then when he got to the station, they were one lb off, then adjust accordingly. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [November 17, 2007, 3:12pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/29 "2007-11-17T15:12:17Z")
My understanding is that it’s within a couple of PSI, and that anybody who has their tyre pressure that accurate is in a minority, and can rest easy. Let’s say you run 32 in the front and 28 in the back, for example. You’ve done your research, and found that’s what you need. You top off the air weekly. You do everything by the book, except for ONE THING: you ignore hot versus cold, and just check the air when it happens to be convenient. I’d say a person in your situation would have safer pressures than Joe “check-every-six-months” Average who has 10psi on one tyre, 35 on another, etc… ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 17, 2007, 9:31pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/30 "2007-11-17T21:31:32Z")
The Car Talk guys contacted tire manufacturers, and the advice is that you are much better off with over inflated tires than you are with under inflated tires. Problems related to heat build up, poor braking/cornering and tire fatigue are all associated with under inflated tires. The consensus is that even at the max pressure printed on the tire, the consequence is a jolty ride, but nothing noteworthy as far as safety is concerned. As a matter of fact, the Car Talk guys have had a long-running issue with another car guy who recommends that you run your tires near the max inflation. Tom and Ray even noted that the guy has data that shows that cars stop better, handle better and get better mileage, and they are actually apologetic for disagreeing with the guy, sticking to the ‘follow the manufactures recommendations’ approach. But more than once, they admit that the info about running tires at the max PSI is pretty impressive, but they just can’t bring themselves to concede. Overall, under inflation is trouble. Over inflation might be better and is only a nuisance if you find the ride jolty. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [November 17, 2007, 10:16pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/31 "2007-11-17T22:16:31Z")
I’d just like to note that if you’re checking your tires, make sure you check your spare at the same time, too. Be sure to examine the side of the spare for info, 'cause if it’s a doughnut, it’s likely to need _60_ psi, not _32_. (If it’s under the back of the car, you might want to also make sure the winch mechanism isn’t rusted, like on my parents minivan. Or that it’s even still there, as the tow-truck driver had some stories where it wasn’t). ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [November 18, 2007, 1:03am UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/32 "2007-11-18T01:03:49Z")
[QUOTE=Sean Factotum] As Hank Hill says, “Always have _two_ calibrated tire pressure gauges with you. Tire pressure is not something you want to monkey around with.” So do you use the pressure recommended by the tire manufacturer, or the pressure on the placard inside the door of your vehicle? Because they don’t always match. [/QUOTE] A man with one tire pressure gauge is sure of the pressures in his tires. A man with two is not sure. To answer the OP, ask the [experts](http://www.rma.org/tire_safety/tire_maintenance_and_safety/tire_safety_brochure/tire_care_and_safety.cfm#inflation) the Rubber Manufactures Association. > [@](#): > > Tires must be checked when they are cold; that is, before they have been run a mile. If you must drive over one mile for air, before you leave home, measure the cold inflation pressure of each tire and record the actual under-inflation amount for each tire. > > Upon arriving at the service station, measure the inflation pressure of each tire again, then inflate the warm tire(s) to a level equal to the warm pressure plus the underinflated amount you measured when the tire was cold. If the cold tire inflation pressure matches the placard recommendation, then no further adjustment is necessary for the warm tire. So if the gas station is less than a mile from your house don’t sweat it. If it is over a mile measure the pressure at home and note just how low each tire is. For illustration let’s say your LF tire is 10 PSI low. When you get to the station, inflate your LF tire 10 additional pounds. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [November 18, 2007, 2:17pm UTC](https://boards.straightdope.com/t/auto-tire-pressures/426596/33 "2007-11-18T14:17:34Z")
[QUOTE=Gary T] There is no pressure recommended by the tire manufacturer. Tire manufacturers specify a MAXIMUM pressure which is not to be exceeded. Auto manufacturers determine the RECOMMENDED pressure for the vehicle/tire combination. [/QUOTE] I changed over to a heavier duty tire on my SUV. BFG AT/KO. 8 ply load range D. In any case, the max pressure is 60psi. Recommended pressure on the car is 35. These tires are quite a bit more heavy duty than say a standard street tire. But not at all unusual for the vehicle. I’m surprised that wildly different tires should have the same pressure. [Previous page](https://boards.straightdope.com/t/auto-tire-pressures/426596.md?page=1)