tire pressure and performance...
 

i drove my new 2004 maxima SE home a bit ago and thought i had made a huge mistake in my purchase as the car didnt handle nearly as well as i had initially thought (and test drove). i got home and began noodling the problem. here it was: the 18" tires came fully inflated to 47psi from the factory. apparantly, the test drive models on the lot had all decreased the pressure to around 32psi and it made A TON of difference. now my question is...i prefer the handling improvement of the lower psi, but will this not ruin my tires far faster than if they are filled to thier recommended pressure? at roughly $300 per tire, i take this issue fairly seriously, and i hope that someone has some good advice or information on this issue.

Yes, reducing tire pressure can reduce tire life noticeably, and you might notice higher gas consumption too (more rolling resistance).

47 psi seems ridiculously high though. Max pressure marked on the tire should not be correct. Are you sure that's what the manual recommends? What size tires? If the pressure is really too high, then your car is resting only on the center of the tread, which would make it wear quickly and unevenly (and handle poorly). Its generally a good thing to have the tread touch the ground evenly. You can check this by drawing lines across your tires with chalk and then driving around. Or just looking at it sometimes works too - but is probably more difficult with low profile tires.

Most passenger car tires that I've seen are at their best around 35PSI (measured when tires are cool). The PSI will go way higher when they've been driven around for an hour. I would definitely not recommend going considerably lower than the recommended tire pressure, as that pressure was chosen for safety, reliability, longevity, and predictibility. If the cold-measured PSI for your car is supposed to be 47, then I'd keep it within 5PSI of that. Anything that much lower will cause them to heat up a lot more than they should and will start to wear a lot faster.

It seems a lot of new cars these days are coming with high pressure tires. I have seen a few 50psi max tires, and several 40-45 max tires. If it was designed for 47 psi... leave it their. Lowering it less than designed will make them wear fast, decrease gas mileage, and make your handling worse. Also, if they are like 35psi tires then they are overinflated. This leads to bad tire wear and sloppy handling also, as well as the car tending to wander. I dont know your car or tires, but if they are standard 35psi recomended, then i would drop the fronts to 37 or so and the backs to 34ish. Their is a lot more weight and force on the front tires on a FWD car, so youll want your front ones a tad higher than the rears.

On the door jam of the drivers door there will be a sticker with the correct tire pressure for that car, check it out and see what the numbers are there.

That is the recomended pressure for your car, what is on the tire is the max pressure rating.

With 18" tires they are probably low profile tires and those are generally high pressure tires also. 43 to 47 is pretty common for the low profile 18s I've seen.

Define what you mean by handling improvement. Typically reduced air pressure may feel less jittery, but sidewall flex is reduced greatly.

Typically, anything but the recomended pressure for the tires or car will decrease performance. So reducing it if its high will improve handling. Same with more air if its low. MY experience has been an underinflated tire gets much more sidewall flex. Lowering the pressure a little will give a larger footprint and more traction, but too much will ruin handling altogether. The less air pressure keeps the tire from holding its shape well, and the sidewalls flex quite a bit on cornering. With increased air pressure you get much less sidewall flex, but you have a smaller footprint, less traction and hold on the road, and subsequently worse handling.

I could be wrong, but the less inflated the tire gets the more sidewall flex you get is what i always thought / noticed.

Not in my experience. The recommended pressure tries to maximize performance, economy, and comfort. If one of these is more important to you than the others, then changing tire pressures can be an improvement. All the cars I've driven a lot have had stock pressures that I felt were a little too low. Increasing the pressure improved the perceived handling qualities, but reduced comfort. (I realize that's going in the opposite direction to what's being asked here).

Warwagon, sidewall flex will be reduced with higher pressure, but if the tires are bouncing off the ground all the time, I wouldn't say handling is improved. The sidewall doesn't even get a chance to try and flex. Even race cars have an optimum pressure that isn't just the highest safe one.

with a typical passenger tire, the max cold pressure is 35 psi. for a slightly more "comfortable" ride, many people opt for 32 psi cold, which is the absolute minimum that these kinds of tires should be run at. its about a 10% difference in psi.

with performance tires, of course, the pressures are, by necessity, going to be higher. but again, the 10% rule applies. if the max pressure is, say, 45 psi, you can reduce it by 10% for a slight comfort improvement. so from 45, you can drop to 40-41. but dont go any lower than that.

remember, both under and over inflation will wearout a tire prematurely, sometimes with catastophic results. keep the psi in that 10% zone at all times.

also remember to make these adjustments with tires that are not hot from driving. adjust pressure BEFORE driving, not after.