Thanks to all who replied to my last post on the subject of tire pressure. I just want to clarify one thing:

If I wanted to inflate my tires to 35 psi, and I check my pressure at a gas station after highway driving, should I inflate to 31 psi? The door tag says to add 4 psi for hot tires.

Thanks again,
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Patrick McHugh, Cincinnati, OH, 1993 244 'classic' model

You asked, "...to inflate my tires to 35 psi [cold],... at a gas station after highway driving, should I inflate to 31 psi? The door tag says to add 4 psi for hot tires...."

Tires increase their pressure as they warm up. What they mean by "add 4psi" is that you should expect the tire to show 4psi more after the tire has been driven (because it's hot) compared to the cold temperature (before starting to drive). So if you want the tire to have a cold pressure of 35psi, you should inflate them to 39psi after they've been driven. They will then return to 35psi when they cool down to their cold temperature.

Also, remember that tires change their cold pressure with ambient temperature changes. As the season changes now and it gets colder, going from summer into winter, tires loose ~1 psi for every 10 degrees Fahrenheit -- that might be a lot of deflation, if you live up north! And vice versa, of course, as you later go from winter to summer!

The inflation pressure on the DOOR sticker apply ONLY to the OE supplied tires and NO others.

The number imbossed on the tire is the "MAX COLD" pressure. It is generally not a good idea to go higher than that COLD.

If you are checking the tires when HOT, then add 4 psi to what you would want COLD. So, if you WANT 35 PSI COLD, that is roughly the same as 39 PSI HOT.
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JohnG 1989 245 MT @ 235,000

The statement that the sticker pressures apply ONLY to the original tires didn't sound right, so I put the question to Goodyear's tech advice email...here's their reply:

"...as long as you have the same size tires....the vehicle manufacturer's recommendations should be followed. The max pressure stamped on a tire has no bearing as to how much air you should use (as long as you don't exceed the max). The same tire may be utilised on any number of different vehicles, and the 'correct' amount of air can vary from vehicle to vehicle. The analogy that is often used is the max pressure on a tire is similar to the max speed on the vehicle's speedometer, in that it may never ever be required...."
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)

Bob,

GY gave you part of the technically complete and correct answer. They are correct as far as they go.

What they FAILED to tell you in that e-mail is that every tire has a particular pressure that is suggested for a particular LOAD. They are assuming (and we all know what that does) that you are replacing like-for-like tires on your vehicle, and that the sepcs for any replacement tires are close to the original. That is a BAD (terrible in fact) assumption.

EVERY tire manufacturer has a "Load inflation Chart" for EVERY tire they make. What GY *should* have done was to ask you which tire of theirs you were asking about, and then send you a copy of the chart for THAT tire. All you need to know then is the load. That information IS on the door panel, and will always be correct (unless you exceede the mfgr weight ratings).

I have tried NUMEROUS times to get these charts from retailers (GY, FS etc, and independants like Tires Plus). All I get is a blank stare, and often the reply of "What, never heard of it." At that point, I WALK OUT.

The tire WHOLESALERS who sell and distribute tires DO have thses sheets, and I have them for the GY tires on my '89. The tires are Invectas (44psi), and to carry the maximum load in the wagon, I NEED 44 PSI. Therefore, the door sticker, for these tires anyway, is ABSOLUTELY WRONG. As an aside, the inflation chart also does not list ANY pressures below 32 psi. Hmm, that's MAX * 0.75. Interesting, the VERY same thing happens to almost EVERY tire on the road.

Exploders. Ford's suggestions are 26 PSI as I recall. The tires "MAX" = 35psi. Hmm, 75% again. AND they FAILED at that pressure. Too much load/speed.

You see, load and speed both do the same thing, generate heat in the casing. If you add load AND speed, you get BOOM! ala: Exploder.

Firestone TOLD FORD that if they insisted on the 26 PSI pressure, that they would need a tire with an additional belt (to reduce casing deflection and therefore heat). Ford rejected this, and people died.

Do what you want (everyone), but I daned sure will err on the side of safety, and keep my tires up to the MAX COLD when cold.
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JohnG 1989 245 MT @ 235,000

I take the OEM tire size, and look up the table (in TIRE GUIDE) for the load rating of that tire at the pressure(s) [viz., f and r, if they're different] specified by the car manufacturer....

[To interject: the thing to understand is that the load rating of any tire varies with pressure -- you want to match the load rating of the original tire to the new tires (of a presumed different size), to optimize ride, handling, and tread life.]

...Then I go to the table for the new tire size that I'm adopting for the car. Look up the same load rating(s) in the table, and then see what pressure(s) provide(s) that(those) load ratings! That's the pressure(s) to use, at least to start, before experimenting for your driving style.

I take the OEM tire size, and look up the table (in TIRE GUIDE) for the load rating of that tire at the pressure(s) [viz., f and r, if they're different] specified by the car manufacturer....

[To interject: the thing to understand is that the load rating of any tire varies with pressure -- you want to match the load rating of the original tire to the new tires (of a presumed different size), to optimize ride, handling, and tread life.]

...Then I go to the table for the new tire size that I'm adopting for the car. Look up the same load rating(s) in the table, and then see what pressure(s) provide(s) that(those) load ratings! That's the pressure(s) to use, at least to start, before experimenting for your driving style.

Tire Guide, an industry annual publication, is in virtually every tire store! Whether they will let you look at it is an altogether other question.

Follow the sticker on the door for any tire but do not inflate the new tire past its max stamped on the side. My tires say 44 psi max. You should inflate your tires cold to ~36 and hot they might be 40. Ken C has explained it correctly. All the new tires will have is the maximum inflation not the recommended. My tire shop (Canadian Tire) inflated mine to 30 when they installed them. The ride sucked and the fuel economy went down 2 MPG.

"Follow the sticker on the door for any tire but do not inflate the new tire past its max stamped on the side."

That is mostly correect. You should not inflate to more than the "MAX" on the sidewall -COLD.

The door sticker is for OE tires ONLY, or their direct replacemets. Do you think that if you replaced the stock tires with some really good high-performance 50 PSI tires you should inflate them to 28 psi just because the door sticker says so? Of course not! Volvo (or whomever) has no way to know what future tire technology will be.

Still think you should follow the vehicle manufacturer's suggestions on inflation and not the TIRE mfgrs? Can you say Explorer/Firestone? Ahh, NOW you are understanding. It was FORD that set the pressures on the sticker too low, and Firestone told them not to. What did you get? Lots of wrecks and injured or killed people, because Ford did not listen to the TIRE mfgr on inflation.

Now, the TIRE mfgrs all know what inflation you need for any given tire at a specific load and speed. Do the auto mfgrs? No. How could they? The vehicle mfgrs are really concerned mostly with two things, ride quality and noise. Higher inflation pressures HURT BOTH. When you are in the business of selling cars, you want GOOD ride and low noise while having good handeling and economy. The vehicle mfgr chooses an inflation pressure that is at best, a compromise. Those pressure may VERY WELL be right at the very lower end of what is safe (Explorer), YOU don't know that, but the tire company does.

So who are YOU going to believe, the auto mfgr, or the TIRE mfgr? If you are having trouble answering that correctly here's a hint... ALL tires explode from UNDER inflation.
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JohnG 1989 245 MT @ 235,000