OEM Tire Pressure Recommendation Question

[Curt C]

The M coupe comes with rear 245/40/17 and front 225/45/17 as we all know. Recommended pressures are 35 R and 32 psi F. A 1999 911 comes with 285/35/18 and 225/40/18. Recommended pressures on this car are 44 and 41. These car weigh about the same. Why the difference - aspect ratio? So I have plus sized my wheels and tires on a 99 M coupe: 285/35/18 and 245/40/18. What pressure should I run? In the middle? Just curious.

[MartyBtoo]

On my Coupe the recommended pressures were 32 square unless you were loading up the trunk, in which case the rear raised to 35. I run 32 square for my strictly street application, and get even tire wear across the treads.

To answer your question, the cars may weigh the same, but the 911 is much more heavily loaded in the rear. Additionally, larger diameter rims tend to have higher pressures; I suspect it is to improve the impact resistance to road hazards with the shorter sidewalls.

Marty

[rf900rkw]

Needed tire pressures vary with tire size and construction, vehicle design and handling considerations. Porsche puts considerable effort into trying to negate a basic design flaw and make the pointier end stay in front of the slightly less pointy end, no matter the ham-fistedness of the operator.

I run 24/22 in my M Roaster and 24/24 in my M coupe. Tire sizes are not stock/

[khammack]

I use the suggested pressure as a starting point and then, over time, I will let the tires tell me if I need more or less pressure. Just keep an eye on your tires and adjust your pressure as your tires wear. Normally, center wear indicates you need less pressure and if the edges are wearing faster than the center, then you need more pressure. Note that mechanical issues can also cause abnormal tire wear.

[kezug]

Yep...it was only a matter of time for another new Z3er to ask... I just put on my Z3 2.8 with type 18 rims 4 Continental ExtremeContsct DWS06 plus tires of spec 225/50 ZR 16 92W M*S tires.
The ZR before the 16 confuses me.


The door guide is more confusing and manual says is another confusing matter.

So, Please help me get the right PSI

All 4 tires are this



Here is the Manual...seems, I have have it right, I need to figure which my tires related to one of 2 possible matches.


I got this from an online manual (assuming this is legit for 1998 Z3)

[Hoyt Clagwell]

I use, 29psi front/32psi rear. Although I'm running staggered rims. 225/45 17 front and 245/40 17 rear both with ZR speed rating. The other size you have highlighted appears to have a Q speed rating (very low) so I assume they are referring to winter tires.