OK, so went out and looked at the tires closer (Michelin Pilot Sports on stock wheels) and for some reason, the prior owner put 285/30 ZR18's on the rear, instead of the proper 295/30 ZR18's. These have a rolling diameter of 24.8" vs 25" for the 295's (from Tire Rack specs). The fronts are the proper 225/40 ZR18 with a rolling diameter of 25.1".

They have been on the car for approximately 2K miles at this point. Is this mismatch going to cause any issues with the viscous coupling in the front differential?

Thanks!

 

Hey Steve, Those rolling distances should be close enough. Porsche recommends staying within 1-2% to avoid coupler issues.

Tim

 

OK, thank Tim! I've been searching thru other threads and couldn't find a definitive answer. I'll swap 'em out for the proper rears when they wear down further...

 

HI,You will have no problems....I have run 285/35/18,25.9'', and 235/40/18,25.4'',for over 22,000 miles with NO problems and Porsche Tech check my car over..They even say 'now' that .5 '' is acceptable..

 

I had Michelin PS2s 225/40 18 on the front at about 30-40% worn and just fit new Sumi lls in a 285/30 18 on the rear. All the specs looked good with published revs per mile of 829 and 834 respectively but the handling went to hell. My PSM would go on if I pushed it in a turn even non aggressively. It scared my wife once on the freeway on a relatively tight turn, the back tire slipped a bit and the PSM braked it instantly without warning. Also the car didn't want to go straight ahead and would continually hunt. It either wanted to turn left or right. If going 65 or above and wanting to make a subtle steering change, the car would turn in way too much and you'd have to correct immediately. Pretty scary and unnerving stuff. I spoke to my alignment shop and they said they could correct it. I then fit some new Sumi ll fronts in a 225/40 18 and all the handling ills went away. The car now handles better than it did with the Michelin PS2s all around with the rears at 50% and the fronts at about 60%+. The handling is better in the cold and wet as well. The Sumis are much more predictable and confidence inspiring as well.

The problem with the mix stemmed from using a very slightly taller tire on the front than the rear. This squatted the rear down a bit and dramatically changed the weight distribution and increased the caster angle at the front. Like driving a truck with a very heavy load in the rear will make the steering hunt. Anyway all is good now.

I'd say if you're too far out of spec, handling deficiencies may surface before you start damaging components. Good luck!

 

Should be fine... your revs per mile are 929 versus 840, so a differnece of 11. 11/829 is about 1.4%.

Think about 315/30 and 235/40 when you eventually re-tire.

A

 

That's the plan!

 

Thanks Nick. I'm wondering though if your ill handling might have been related to different tire makes front and rear? I've always thought this was a no-no, and more so with the AWD?

I believe the factory fitment is VERY slightly taller in the front (like .1").

 

Mismatched sizes and brands = no bueno