Infiniti recommends 18 psi below the max tire pressure?

[TooMuchGlass]

I have a 2008 Infiniti EX35 with dunlop tires on it. The tire sidewall says the max is 51 psi (it also says something about 40 psi in really small type but I think it has something to do with the bead sealer). That helpful little sticker on the inside of my door says I should inflate to 33. I took the car to a place that agreed to fill up my tire for free, cause I had one low (the tires take nitrogen. While I'm not convinced nitrogen is necessary, I'm not confident enough in that yet to just fill them up with good ole' atmosphere.) and the guy went on and on about how it would be a terrible idea to fill the tires up above 38. (I thought nitrogen was supposed to help with that. . . )
Anyway, I've read the posts in the corral about nitrogen- I'm not trying to end up there! I'm asking about tire pressure. I'm no expert when it comes to cars, but I do know that riding my bicycle sucks when I have 70 psi instead of 120. Doesn't my car feel the same?

[Daox]

In short, yes.

31 psi is to give you a nice comfy ride. Autocross racers run more like 45 psi for better handling. We do the same, but because it lowers the rolling resistance of the tire (just like your bike tire). Sorry, your tire guy has no idea what he is talking about and is probably just regurgitating what some sales guy spewed out. The fact is the tire can easily take 51 psi without any issues what-so-ever. You will notice a harsher ride, but IMO its quite livable. I run all my tires around 50 psi.

BTW, no, nitrogen is not necessary at all. No tire requires it. Its just a money making gimmick.

[TooMuchGlass]

Awesome, Thanks Daox.
So filling up with normal air to 50 is what I'll do then!
While you're here- the tires are quite loud. Do you think the increased pressure will quiet them?

[Daox]

I doubt it. That is more a tire tread/material/design issue.

[TooMuchGlass]

Hmm, ok. Well it's something to ask a different tire guy in my area about when the time comes.

[serialk11r]

Lower pressure increases contact patch size too and gives you more grip, I think racecars typically run low tire pressures (although they are wearing rather different tires from our cars). Of course since the tire has some degree of structural integrity of its own so it's not the linear relationship some people claim it to be. However the sidewall deforms more when you reduce pressure and that's said to "reduce feel" among other things.

My car has 26psi front 32psi rear written on it, and factory suspension is not the most comfy thing ever, but after I increased the pressure slightly I think it's noticably less comfy.

[PaleMelanesian]

How new / old are these tires? Most new tires are quiet, most old tires are loud. (based on personal experience with Goodyear, Michelin and Firestone)

[TooMuchGlass]

O yeah, thanks for the reminder killer. That was the other thing the tire guy was quick to tell me. He said if I were to inflate to 51, I would wear out the center of the tire really quickly. I can see on a theoretical level how that could be true, but practically, does that happen? Or, as Daox suggested, is it just more regurgitation?

[Sven7]

Drag racers run lower pressure because of the larger contact patch but autocrossers and circuit racers pump em up to reduce sidewall roll (when the tires tuck under a bit during cornering, as below) and improve handling.



Since most rolling resistance is because of sidewall flex (in a straight line or not), inflating to a higher pressure will help you roll better.

If you overinflate the tires, yes, the tread will bulge in the middle. The opposite is true if you underinflate them. However at max sidewall pressure or a few psi off you'll do no damage. I'm going to go out on a limb and say you'd have to be pretty far off the recommended numbers and/or do it for a long period to get abnormal wear on your tires.

[TooMuchGlass]

PaleMelanesian: My wife's father changed the tires prior to our wedding. I think he changed them every 25k, and the car has about 65k on it now. So I'm guessing about 15k. But last time he was riding in the car he mentioned the tires being loud like they just always are, no matter what. I just figured they were loud because they're wide with low pressure.