Okay, just about anywhere you ask this question, the answer is pretty clear:

"Do I go by the pressure number written on the car's door frame, or the pressure number written on the side of the tire?" The answer is always: "Use the number on the door frame. The number on the side of the tire is a maximum do-not-exceed number."

Okay, but here's my follow-up question that I can't seem to find the answer to anywhere:

How can this "door frame" number be true for every make and model of tire I put on the vehicle? You mean to tell me, that among all of the varied brands and types of passenger car tires available, that they all need to be inflated to the exact same pressure as the tires that came on the car when it was brand new?

You'd think there'd be some variation among brands and models, wouldn't you?

The reason I ask is because I just put new tires on my car, and as usual, the tire shop inflated them to 35psi, when my door frame clearly says 30. And even at 35psi, these new tires still appear to have a pretty significant pot-belly bulge next to the contact patch. So is my door frame still right?

Now, before I get a lecture on the pros and cons of different tire pressures, I already know most of them:

- Lower tire pressure will give a smoother ride and greater traction (contact patch is bigger), but will cause the tires to heat more quickly and wear out more quickly.

- Lower tire pressure will cause the tires to wear more quickly on the outside edges than in the center.

- Higher tire pressure will cause the tires to wear more quickly in the center.

- Higher tire pressure will make the car's handling appear to be more "darty" and "oversensitive" because the contact patch is smaller.

Really, I just want to know how that door-frame number can really be right for every single tire I could possibly put on the car.

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Really, I just want to know how that door-frame number can really be right for every single tire I could possibly put on the car.

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Because the same volume of air in the tire (if you stick with the same size & profile) has to support the same car weight, which requires the same amount of air pressure.

Really, I just want to know how that door-frame number can really be right for every single tire I could possibly put on the car.

Well, the problem arises when you try and define what is "right".

It depends on what you are trying to accomplish. Are you trying to maximize tire life? Are you trying to get the most comfortable ride? Are you looking for the best adhesion? Do you want the most responsive handling? Maybe you are trying to optimize fuel mileage?

The truth is, no one tire pressure will be best for any more than one of the above requirements. The number posted on your door-frame is the manufacturer's best guess compromise to at least try and satisfy some of them based on the weight of your vehicle.

If you are using the same size tire as recommended by the manufacturer, there will probably not be so much difference in construction from one brand to the next to warrant much concern that tire "A" has inflation requirements different from tire "B".

tanstaafl.

HI!

The answer to your question is that you are supposed to choose a tire which has the same sidewall stiffness as the original tire. Higher speed rated/performance rated tires have a stiffer sidewall to generate less flexing while cornering, accelerating and braking. If you decide to go with a cheaper tire which has a lower rating, eg. from a z rated to a T rated, then the tire is going to need more pressure to make up for the softer sidewall..

How do you think they make 'run flats'? They just have an extremely stiff sidewall, and are usually mounted on high performance cars. :-))

Just eye it.

Yeah, what he said

There is no "right". There's just a wide range of aceptable. Somewhere, the tire life / traction / mileage / snap handling / constant cornering lines all converge on the graph at some psi that you can accept. Not that you have such a graph, but if you did... Also remember that Honda set those numbers based on the "average driver" in their target market, and not necessarily the way YOU drive.

Myself, I'd start at the mfr recommendation on the door sill. On the fronts, I'd add or subtract depending on how I felt about a few things: 1) Does it sound like an overinflated basketball if I hit a bump int he road - a "ping" of sorts? 2) Do I get good grip accelerating? 3) Is the tread wearing evenly? etc. For the rear, once I have the fronts dialed in, I'd go with whatever gives good handling. Too much air may make the backend loose. Not enough will stick it at the expense of lots of understeer.

Of course every car, every driver, and every situation will vary. How's that for a cop-out? But it looks like you are well versed in each pro and con. At this point all I can think of is - experiment!

Thanks, guys. I understand the trade-offs between comfort, handling, and tire life. My goal in this case is to get the best tire life (i.e., the tire wears evenly from center to edge), as the other factors are fine whereever they fall (this is a passenger car that doesn't get cornered hard).

However, I now have two contradicting answers to my main question.

One answer said that the same volume of air, supporting the same weight, will need to be the same pressure. Which makes sense. But then, there is also the answer that says the sidewall stiffness is a factor, and that's the thing that I'd expect to vary from tire to tire. And that also makes sense to me. So I'm still confused.

I've gone through all this before and I came up with one answer: stick close to the car manufacturers recommended pressure, maybe -/+ 2 psi depending on how it feels to you. There are all sorts of ways of "tuning" your pressure to get optimal settings, but for most every-day drivers it's not worth the effort. In the end, somewhere between 28-35 is optimal.

Just my $0.02....

Dont go near the maximum do not exceed number. Stick to the door numbers more or less. Obviously there is variation due to any number of factors. On my car, the door has different pressures listed for front versus back. And I in general run the pressures higher, so the tire doesn't roll during very hard turns.

Calvin

Tyre contact patch is determined by the tyre design. Go to the Michelin website for an explanation. It has very little to do with operating pressure or sidewall stiffness, or any other structural factor.

The weight of the car determines tyre pressure. Since the weight doesn't vary by very much, then neither should the tyre pressure. Rubber has a given coefficient of friction against a road surface, and this determines grip. If a car of a given weight needs a certain "feel" or "grip" then the designer looks at a manufacturer's catalogue, looks at the models available, then chooses a tyre in combination with a given wheel size and rim width that offers the largest/most economical/whatever contact patch possible for his design. He then looks at the all-up weight of his car, looks up the manufacturer's recommended tyre pressure for that tyre, does testing, tweaks it until it feels OK, and then repeats with many different makes of tyre of the same design characteristics. He then specifies a pressure that makes all the tyres of the given design size work properly for that application.

When the car is rolling, then there is a measure of force applied downward by the atmosphere acting on the bodywork, hence the effective weight of the vehicle increases. Hence, for constant high speed motoring, you increase tyre pressures.

Tyre wear and handling are affected when you allow the pressures to very by more than 10% above or below the manufacturer's (car, that is) recommended pressure. However, far more rapid wear is usually caused by your wheel alignment being incorrect, or how hard you corner (front wheel drive vehicles, mainly). If you kept your tyres 25% over pressure for a year, you may begin to notice crown wear. If your alignment was out by a degree or so for just a month or two you would see significant shoulder wear. Of the two, high tyre pressures would as like as not kill you in the first wet weather heavy braking you encounter.

Tyre weight, sidewall stiffness, carcass construction and all the rest do not generally affect the handlng characteristics of a tyre - otherwise tyres would vary in behaviour between manafacturers or be dangerous to use, and manufacturers would rapidly go out of business. The tyre's structural behaviour is effectively that of a three-dimensional spring, whose characteristics are largely determined by the volume (and therefore mass) of air they contain.

The number of plies in a crown or in the sidewall usually determines the tyre's ability to resist deformation on hard rim impacts, and resist penetration leading to punctures. A rubber-only crown, for instance, would puncture rapidly and would as likely or not burst instantly: a ply protected crown will resist a puncture, and even if punched clean through the ply layers, the plies would prevent the crown rupturing from internal pressure. Instead, the tyre deflates gracefully giving you more time to come to a halt. And before you start saying that the tyre will come off the rim if this happens, no it probably won't because the rim is designed to stop this (J type and K type are, anyway) and the steel bead around the foot of the tyre wall is re-inforced, usually welded steel made to a close tolerances and high quality standards.

Don't play around with tyre pressures in a misguided attempt to make the car "handle better" or some such. It will bite you back. You need to look at your suspension as well, not just the tyre. Don't forget, one of the first things the coppers do at a major accident site is measure the tyre track lengths, look up the coefficient of friction for that piece of road surface, and then take the tyre pressures of the crashed vehicles. It all goes into the accident report: some insurance companies will invalidate your insurance cover in the event that there is evidence of incorrect tyre pressures (no joke, this).

Moral of the story:

- Stick to the CAR manufacturer's recommended tyre pressures, for all speed conditions
- check your tyre condition and pressures regularly
- don't play about with safety critical equipment, especially if your family or other passengers are travelling in the car.

"Tyre weight, sidewall stiffness, carcass construction and all the rest do not generally affect the handlng characteristics of a tyre"

I totally disagree. You are telling us that sidewall stiffness does not "generally" affect the handling characteristics of a TIRE? Total Bullshit.

So, if it's all in the 'so called' tire pressure, then why did they need to invent run flats? That tire depends ENTIRELY on it's sidewall stiffness and drives just as well with NO pressure as it does with pressure, you just can't go as fast.

Also, as long as you don't drive a North American car with Super bubbly assed tires on it, you definately feel a difference between tires with stiffer sidewalls or softer sidewalls. The stiffer sidewall doesn't allow the tire to roll laterally from under the rim, which most definately affects the cars handling.

Wow. Cool. Thanks.

In performance circles, the tire sidewall is considered to be a part of the suspension. The flexible sidewall acts exactly like a spring. So in effect it's an extra spring in the suspension system. If you have a very rigid sidewall, then it makes your suspension stiffer. If you have a "bubbly assed" sidewall then your car will behave as if there is an undamped spring in the system. I think this *would* affect the handling of the car, but would it affect the handling of the tire? I don't know.

If you lose all the air in a run-flat, sure you can keep going at 55, but that doesn't mean you can run the slalom in one with equivalent performance.

Calvin

The method I learned was simple. It sure sounds reasonable anyway.

Find a relatively level and flat surface, a large one. Start with over-inflated tires that are cold. (having a portable pump and accurate gauge is handy as well) Scribe a chalk mark across the tread of each tire. Drive a short distance without turning and without hard braking and check the chalk marks.

Chalk worn off the center of the tread indicates over-inflation. Off the edges, under-inflation. Patiently add or remove air and rescribe the chalk marks until you get even wear of the marks across the entire tread of each tire.

This should result in a tire pressure that maximizes the contact patch for each tire. Don't forget to write down the optimal pressure settings.

Oh yeah, this is my first post, so a
Hello World is in order.
I've done enough lurking, so here I am.

Wow, that sounds like a very accurate way to determine tire pressure. And also a hell of a lot of trouble!

And welcome.

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Wow, that sounds like a very accurate way to determine tire pressure. And also a hell of a lot of trouble!

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It sure sounds accurate to me, though I'd be interested in any refutation of the method.
And yes, it is a PITA to do. But how else to figure pressure on oversize tires? My stock tires had the standard 35 psi max rating, but the ones I have now are 50 psi max. Using the manufacturers spec pressure (26) made the tires look kinda flat. Not good.

Yeah, chalk comes in pretty handy for that. It's also cool for seeing how far you roll over on the sidewall under extreme cornering. Even cooler than chalk though, is using a laser thermometer. I've seen dudes at autocrosses do that a couple of times. Basically, they do a few laps, and then check the temperature accross various parts of the tire.

Yeah, I guess that is a little out of control. But hey, those guys are sponsored.