MrIncredible

For what its worth, I do like the wheels.

Frank 993 C4S

Not easy to diagnose with that description but I would check:

1) alignment
2) worn suspension components

If the car is lowered quite a bit, it could also be bump steer.

M3Cavalry

I went to reduce my tire pressures as you all recommended, and to my surprise they were already nearly where I wanted them (32/38, cold). I set them to precisely 32 and 38 using a handheld Intercomp gauge. My TPMS reads +4 psi for all four tires. I reset the TPMS for good measure and it continued to read +4 from the handheld gauge for all four. I have 245/30/20 and 305/25/20 tires. The TPS does not have a setting for this, so I kept it at the 19” summer tire setting.

Over the course of the drive, the rears got to 49 psi indicated. That seems crazy high.

Should I be suspicious of my trusty handheld gauge? Reduce pressure further to 30/35?

Wayne Smith

My rear tires can easily increase 12 psi when I'm having fun.

Rig.Stunts

I'll expect + 6 psi, even at the track. 49 psi, and + 4 TSPM reading, something seems off

ADias

In no circumstance should summer tires be much above 40PSI when hot… 39PSI max for me.

M3Cavalry

Just ordered a digital tire gauge to check against my trusty Intercomp.

Wayne Smith

I thought the same. Went to nitrogen fill (abouts water vapor). Same thing. Measured as accurate by three different tire gauges. I've always been a mid engine driver. I figure it's additional weight based. But running the universal gas law ... to get a 25% increase in pressure would require a 100F increase in temperature (approximate, if course). I haven't read temperatures.

Note: Front tires don't see these big increases, in my experience. Also, driving afternoon or morning with sun favoring one side of the car, the pressure on the sunny side can be 2 psi higher. Turn around and drive the other way on the high side will flip.

Another note, 2010 C4S manufactured August 2009, still on the original sensors and going strong. Yeah, that's past the shelf life of most batteries.

M3Cavalry

that’s the one!

NevilleNobody

I'd consider going back to a 19 inch wheel, tbh 20s never look right on these cars imho and from there you can start to undo the mods to some degree.

M3Cavalry

I agree. That is the plan!

ADias

Rear tires very close to a very hot mass, especially during spirited drives. That’s why I now set rears at 34PSI cold (similar directions for newer cars and GT3s).

Wayne Smith

I've always followed your mandate that the rears should be at least 5psi over the fronts, but what you've written here makes sense. Thanks for sharing your thoughts.

Bruce In Philly

I went from S-standard 19" wheels to 18" non-S, S-winter size because I got sick of repairing my 19" rims... they bent constantly... one time so badly, I hit a pothole on the interstate and blew out/destroyed two tires, that I was worried about metal fatigue. It is amazing what wheel repair places can do.... I watched one place work and saw huge flames around someone's rim. I think my car looks fine... actually, I like the look of wider rubber... maybe that is my age but I really dislike these rubberbands-over barrel wheels... but that is just me.

Peace
Bruce in Philly (now Atlanta)

ADias

My mandate?!? LOL

5PSI differential may be too high. Recent recs from PAG bring that down to about 3PSI - check 991/992/GT3 PSI plates. I use 31/34PSI cold.