tire pressure question

[Mr.Stabby]

alright, this may be a really bad question but i am gonna shoot anyway.

if you change your wheels from stock to a larger rim and lower profile tires, would you fill the tires the same pressure if they were stock ex. 32psi.

i uno i was just wondering.............:rolleyes

[GunnerNell]

Actually, I think that's a good question... at least, it's one to which I don't know the answer. I'd guess it's the same, but I'd like to hear from some experts, which obviously I am not.

[mryakan]

If you notice, the OEM tire inflation specs differ by tire size or spec, so that is a valid question. Usually it is a good idea to stick to recommended OEM sizes, to keep the rolling circumference as close to OEM as possible and in that case you will find a matching tire inflation recommendation on the door sill. However, sometimes you can go with non OEM sizes and in that case I have no clue how to adjust the pressure, I would guess people start with OEM and then play it by ear. Any size in particular you have in mindÉ

[zonerlbc]

Like mryakan said- inflation specs differ by size and spec. I always check to see what the recommended inflation printed on the tire is and go from there. Just always make sure to check the pressure when the tires are cold.

[GunnerNell]

There's a recommended pressure stamped on the tire itself? The only pressure I recall seeing right on the tire is the maximum pressure. I'm a proponent of running higher pressures than the owner's manual calls for because I believe manufacturers generally recommend an artificially low pressure in an effort to "improve" ride quality. But even I would never suggest pumping them up to the maximum rated pressure. Yikes.

[mryakan]

There's a recommended pressure stamped on the tire itself? The only pressure I recall seeing right on the tire is the maximum pressure. I'm a proponent of running higher pressures than the owner's manual calls for because I believe manufacturers generally recommend an artificially low pressure in an effort to "improve" ride quality. But even I would never suggest pumping them up to the maximum rated pressure. Yikes.
Not necessarily. Some may recommend tire pressure to improve gas mileage, others may recommend ones to increase tire longevity, while some may recommend ones to improve steering response, among many other reasons. It all depends on the manufacturer's philosophy, but one thing is for sure, they test those recommended pressures extensively. One can always increase/decrease the recommended pressure, but something will be sacrificed (faster or un/even wear, less/more steering response, better/less traction, softer ride, etc.), but that requires quite a bit of experimentation and some of it may be costly esp. if you end up wearing your tires prematurely. Most people will just stick to the recommended values and some may go with previous personal experience or advice.

[GunnerNell]

Although I don't have any technical training or knowledge in this area, I've been driving for over 45 years, so I guess that's some experience. Long ago, someone who did have the training/knowledge told me the stuff about manufacturers recommending lower pressures for ride quality, and later, that apparently was one of the things that got Ford/Firestone into trouble. Still, I hear you about the possibility that some would recommend higher pressures for, say, better gas mileage. I tend to run slightly higher pressures than the recommended ones, but I know I get a "thumpier" ride as a result. I think I get better performance, gas mileage and tire wear that way, too, but that's based more on a seat-of-the-pants feeling than empirical evidence.

[Mr.Stabby]

alright well if it helps, the tire shop in which they were purchased filled them up to 45 psi........ so í do believe i should lower them but not sure yet... there should be some sort of equation to figure out by tire size and car weight.

[zonerlbc]

I've noticed that a lot of tire shops will overinflate the tires, but I'm not exactly sure why. A car is certainly easier to steer and has less rolling resistance with an overinflated tire, but that is because the tire has a smaller contact patch with the higher pressure.

I only use the max. pressure rating as a guideline and usually run a few or more pounds below it. Pressure is a function of the tire itself, the compound, aspect ratio, sidewall stiffness, etc. Run-flat tires are going to behave differently than a touring tire. However, if a tire states a maximum pressure of 45-50 lbs., I would think that 32 lbs. is going to be a little underinflated...

[asianvenom]

Alot of shops over inflate the tires because they put air in the tires, before they put the tires/wheels on the car. You see the problem here? the tires are not loaded, so when they put the tires/wheels on the car with the car weight, now it becomes over inflated.

[Mr.Stabby]

soo.... what would you think?? 235/45-17 tire on a approx 3800lb car? what would you fill it to?

[mryakan]

Alot of shops over inflate the tires because they put air in the tires, before they put the tires/wheels on the car. You see the problem here? the tires are not loaded, so when they put the tires/wheels on the car with the car weight, now it becomes over inflated.
huh? The pressure is the same whether the tire is on the car or not.

[sixcyl320i]

the tire pressure gets different when you squeaze it man comeon.... :shifty

kieferstr Id run 36-38. maybe lower depending on the tire. At work all tires i fill are set to MFG reccomended pressure, and there seems to be no tire wear issues due to inflation pressure. Some of the tires i see with too much pressure wear out in the center. for example 40 psi is too much for a saab 9-5sdn, but 35 is fine.215/55 r16 ..on my e36 I run 30 front 27 rear, but im usually trying to get my tires to heat up some :D

[GunnerNell]

I'm with Mryakan on this. Your logic isn't wrong, sixcyl, but it is incomplete. Dropping a car off the jacks, onto the bottoms of the tires, doesn't equate with squeezing the tires from all sides/angles. I'm sure you have "squozen" (how d'ya like that one -- made it up m'self!) a few balloons in your day, and what happened? Squeeze a balloon from left and right, and it just expands front and back; interior pressure does not increase, it just moves to other parts of the balloon. And yeah, over-pressurizing will wear out the center tread, but I think the steel belts give us a little wider window to work with.

[sixcyl320i]

lol... i was kidding with the squoze comment. but thats ok.. ill give it one more shifty

:shifty

[Mr.Stabby]

alrght thank for the help you guys! at 45 psi the tires arent bulging in the middle, or is there a flat spot on the bottom of the tire (ya know... like where the sides are bulging out where the car sits) but 45 does seem a bit too high so might as well lower it see what happens. but i gotta agree with asianvenom on this one. the wheel mounted on the car should add more pressure due to the added weight from the vehicle (just like stepping on a 2L empty bottle noticing that there is obviously more pressure inside then if you are not stepping on it)

[GunnerNell]

Again, incomplete logic and a faulty comparison. That 2-liter bottle has only 1 atmosphere of pressure in it, and I doubt it could be pressurized sufficiently to where it could support your weight. Not so a tire. And by the way, I didn't mean to say that the weight of the car has nothing to do with this, only that it wouldn't appreciably raise the psi's inside the tires, from a proper to an over-pressurized level.

[M397]

If you have too much air pressure on low profile tires you tires will wear out excessively from the tread. Under inflation will cause your sipes and shoulders to wear out faster than your tread. If your running 245/40/17 on your e36 which is a stock fitment, just put 32 pounds. If you were running 20 inch rims I suggest going with 40 pounds of air, according to the tires specification.

[ddsski]

Recommended tire pressures are the optimum blend of ride, wear, handling.

To overinflate a tire to be driven on the street (say 40psiv. stock 32) you will see the following happen. Ride deteriorates, wear chews up center of tire (ask any Elise owner about this) but handling is more subjective. Steering effort is lower and car will probably be better in the rain due to smaller, more highly loaded contact patch, but I'll bet you'll find braking distances go up in the dry for the same reason. Nowadays, all manufacturers are trying to max their mileage numbers so I doubt you'll ever see the Ford (their fault IMHO) debacle again (remember Firestone said tires should run at 30 psi but Ford insisted on 26PSI) , but I digress. Maximizing handling wet .v dry is usually the key criteria now used, I believe. Ask the experts at the Rack on this one.

[Mr.Stabby]

wow i'm surprised thjis thread has been going so well... i just really did not want to ask a "how much do i fill my tires" question without looking as stupid as novking3 who started that 17 page argument....HA!

[GunnerNell]

Not so fast there, Pilgrim. Nobody said you didn't look stupid! :D

Kidding.

[era87]

Well here's my take, On my e46 I run 235/30/19 Kumho AST's in the front with 32-34psi and in the back 265/30/19 Kumho AST's with depending on weather really about 36-39psi..

Really depends on size and tire you choose.. For RWD its different then AWD.. RWD will always require a little more air in the rear tires if you change them.. Could go up to 42psi.. with the front staying about the same..

[Mr.Stabby]

alright i guess my worry is having a blowout from being under/over inflated. when they were put on they were sitting at 45psi. lowered them to 40 but i believe i am going to lower them more for a smoother ride.