Stupid tire question

Just put 2 new tires on the V. PO had the pressures at 32 all around. Car felt a little squirmy and fronts would feel flat-spotted for the first 15 miles or so. Tire shop inflated the 2 new ones to 39...between that and the fact that they're new, it's a night & day difference. I just bumped the rears up to 40. Haven't driven on them yet, but noticed that the MAX pressure is 51. Should I bump them up more? Tires are W rated Godyear Eagle GT's. Google is giving me nothing relevant.

As long as its within the limits of what the tire sidewall reccomends...do whatever you want. if you care about tire wear just keep an eye on them and try to find that sweet spot of even wear and non-mushy sidewalls and you are good to go.

on my DD I almost always have the tires several PSI above what is reccomended on the door of the car. (because what the car manufacturer reccomends is great for comfort and very squirmy when you toss the car into a turn)

my doors say 32 PSI all around, I run 36-40 usually. (on a 2500lb 100hp hatchback)

Good lord, man! Have you never read Hunter S. Thompson?

As long as the tires are designed by Sandoz Laboratories, you could load them up to a hundred! You will reap the benefits of superior handling.

"The Whale behaved very nicely with the altered tire pressures. The ride was a trifle rough; I could feel every pebble on the highway, like being on roller skates in a gravel pit… but the thing began cornering in a very stylish manner, very much like driving a motorcycle at top speed in a hard rain: one slip and ZANG, over the high side, cartwheeling across the landscape with your head in your hands."

from Fear and Loathing in Las Vegas

My door sticker says 33, but I run them at 36.

You know, I was just having this same discussion in my head. My Khumos were down just under 28psi on a below-freezing mornign and I ran them up to 32 all around. When the day warmed up the car drove a LOT better, you could feel the tire pressure had come up. Suddenly the car was sharp and fun like it used to be on my old tires. I haven't checked to see how high, but I'm betting daytime pressures are between 35 and 40psi.

I was going to query the hive mind about what tire pressures ppl were running in the DD.

Regardless of the tire type, size, or max pressure, the sticker on the door should be your starting point.

With obvious slight differences in sidewall height and stiffness, and a few other factors, what supports the weight of the car is almost entirely the air pressure. Proper air pressure is key. The sticker on the door is correct for the car. The max inflation pressure is used to determine weight capacity. If you have a tire with a max inflation pressure of 40psi, it won't have a corresponding weight capacity to make it suitable for a 1-ton dually.

The tire's job is to contain air pressure and structure. The air does all of the support.

So for instance, lets say you started with 255/50-17, and swapped to 255/30-20, it doesn't matter.... the proper pressure should be about the same. You're still supporting the same weight. The tire is just there to hold the same XX psi that is required to support the weight of the vehicle.

The best way to quickly determine proper pressure is to start with what the door sticker says. Now burn some rubber. If the black patch is darker in the middle, the pressure is too high. If the black patch is darker at the edges, the pressure is too low. You're looking for a good even darkness across the contact patch.

But, you'll probably find that the door sticker is almost always spot on, regardless of the tire.

All I have to add is that I take what's on the door sticker as the absolute minimum useful tire pressure.

Edit: That's for passenger car tires, not trucks.

from a static point of view, yes, psi is psi and all tires are pretty much the same.

But when tires start moving, the dynamic properties are the result of the tire's construction. Those dynamic properties are affected by the pressure in the tire not just at the contact patch but elsewhere as well - the sidewalls and even the tread at the top of the tire. If this were not the case than any two tires of the same size and load rating would be indistinguishable in handling characteristics at a given pressure, which doesn't happen.

You're probably right that starting at the door listed pressure is the best idea, and I'd bet most people won't gain much by straying too far. But I still think that tire pressure is the most overlooked handling tuning option by the average driver.

Learn something new every day! I've always run 40 on my FWD hatches with good results (even wear, good FE.) 40 feels grear in the fronts, as I'm guessing it will in the rears.

Thanks for the info. I'm a dumbass.

poopshovel wrote: Learn something new every day! I've always run 40 on my FWD hatches with good results (even wear, good FE.) 40 feels grear in the fronts, as I'm guessing it will in the rears. Thanks for the info. I'm a dumbass.

When tires are being burned, if they go "pop thump thump thump" you were burning your tires too much. Ask me how I know.

I have the sneaking suspicion FE will get a slight improvement as well. Car definitely felt like it was rolling over the sidewalls at 30.

There's a bit of a curve, isn't there?

I have it in my head that you can throw away traction (like you might to for the rear of a FWD car if you're out of options) via either under-inflation or over-inflation...

Am I making stuff up? Having those pesky memories of things that never happened? It would seem rooted in truth for the same reason that over-inflation will wear the tread center faster; you're not planting the shoulders as well, basically picking them off the pavement by the tread center...

FWIW (not much), 40 doesn't sound crazy high to me.

I remember reading on a MINI forum somewhere that rear tire inflation was commonly used to induce over steer. Lower than normal pressures on the rear gave a slower, more progressive breakaway, while high pressures gave a quicker, snap breakaway. Supposedly because of this open track guys in stock minis usually ran rear tire pressure on the low side while autocrossers liked them high.

I do know the one time I autox'd my wife's stock Cooper S I ran the rears high (45?) and managed to get a tank slapper going on my run through the slalom. I backed off a little and it became manageable. Also, 5 psi change in my Mustang tires can make noticeable difference in autox as well.

FWIW (also not much,) yes. In a FWD (which is what I'm used to,) I'd play with the pressures quite a bit on street tires to get the car to rotate. A pushy pig of an integra at 35 all around becomes a trail-braking induced oversteering joy to drive with 40 up front & 45 in the rear.

I had the same tires on a Honda Fit. Autocrossed it a few times, determined 36-40 psi was optimal for handling.

HappyAndy wrote: All I have to add is that I take what's on the door sticker as the absolute minimum useful tire pressure. Edit: That's for passenger car tires, not trucks.

This. I've got my second set of General Altimax HP's on the MPV. The first set was great, just wore out the shoulders long before the middle was even close. That was running them at the door sticker pressure (give or take) which is 36 F and R. This set I'm trying to keep between 40 and 45. Hoping that will even out the wear.

Ransom wrote: There's a bit of a curve, isn't there? I have it in my head that you can throw away traction (like you might to for the rear of a FWD car if you're out of options) via either under-inflation or over-inflation... Am I making stuff up? Having those pesky memories of things that never happened? It would seem rooted in truth for the same reason that over-inflation will wear the tread center faster; you're not planting the shoulders as well, basically picking them off the pavement by the tread center...

these days it probably comes down to.."it depends on the tire"

I put 50k miles on a set of tires, daily driving and running 8psi over manufacturer reccomended in the front, and 4 over reccomended in the rear, and they wore perfectly even.

I consider the sticker on the door a minimum, and on anything prior to 90 or so completely useless. Grab any new tire with a max load at pressure of 50 psi and compare it to a 20 year old tire with a max load at pressure of 32 psi and the new tire will weigh less and it's going to significantly softer and thinner sidewalls.

With the newer tires you need higher pressures to keep the less stiff sidewalls from flexing. That flexing builds up heat in the tire. That max load at pressure corresponds to the pressure where the tire builds up the least heat and/or dissipates the heat the best. Ignoring it for a sticker is dumb.

If your sticker says 32 and the original tires had a max load at pressure of 36, then it would make more sense for you to start your new max load at 50 pressure tires at (32/36)*50=44#. That would be my starting point.

curtis73 wrote: Regardless of the tire type, size, or max pressure, the sticker on the door should be your starting point. With obvious slight differences in sidewall height and stiffness, and a few other factors, what supports the weight of the car is almost entirely the air pressure. Proper air pressure is key. The sticker on the door is correct for the car. The max inflation pressure is used to determine weight capacity. If you have a tire with a max inflation pressure of 40psi, it won't have a corresponding weight capacity to make it suitable for a 1-ton dually. The tire's job is to contain air pressure and structure. The air does all of the support.

In a radial tire, the tire carcass and sidewall provide a significant amount of support. Air pressure does provide much of the support, but the contribution of the sidewall is not insignificant. The air pressure needs to keep the tire carcass in the correct shape for the sidewall to carry load.

Consider the construction of a runflat tire. They aren't that much different than normal radial tires. There is some thicker rubber and more reinforcement on the base of the sidewall around the tread, so the tire sidewall can't completely fold over when unsupported by air pressure. The tire supports the load of the vehicle with no air pressure without collapsing flat.

Bear in mind that on no OEM tires, the tires may haze different strength capabilities let's say the OEM tire had a 101Y load rating and you replaced it with a 99W; the 99W would not be as stiff and as a result, require more pressure.

Higher tire pressure also help significantly in rain and snow; helpful if you like to get every last nickel out of them.

I have always went with lower rear tire pressures to induce oversteer with the ideaology that peak grip will occur at a higher slip rate and it will be more forgiving when it does start to slip. The more air you put in a tire, the lower the slip rate for peak grip and the quicker the grip "falls off".

If I expect signficant or prolonged periods of snow or if I'm running tires with low tread depth (<3/32"), I bump the tires up to the max pressure to make the tire narrower and the tread more pronounced and in some cases I'll set the fronts to 4psi bellow the max pressure to avoid oversteer. Off-reader only lower the tire pressure to avoid "sinking" and for more compliance on rocks, etc.

Feel free to play up to 51 but bear in mind that the tire is more likely to go pop over a pot hole especially if they are low profile tires. I've went over the max pressure before but only on a smooth autocross course; in a rental car.

Check out the owners manual. I believe there is a part in there that talks about what psi to run bepending on how fast you are going to go. I have used the fully loaded psi on the door sticker for all of my cars for a while with good results.