LADO

Question about tire sizes

I’m currently running 225/40/18 front and 295/30/18 rear. 04 4s

Little bit of math:

225 / 100 x 40 = 90mm is the height of front tires

295 / 100 X 30 = 88.5mm is rears.

Perhaps I’m changing them with higher profiles tires

Fronts 225/45/18

And rears 295/35/18

It means that if previously fronts was higher 90mm and rear 88.5mm now I will have 101.25mm fronts and 103.25 rears. So if previously my front tires were making less rounds now rears are making less.

If it was rear wheel driving car without ABS and PSM it would be ok, but my car is 4wd with ABS and PSM so can I make this change? Will it run ok? Has anybody done that before? If this tire sizes are not suitable for my car than can anybody advice what set of higher tires can I put on???

SportsCarAdvisor

BUMP, I'm looking for same question/answer.

I’m currently running 225/40/18 front and 295/30/18 rear. 2004 C4S

and wondering what the impact is if any on driveline / AWD systems, speedo guage systems etc. if I switch to:

Fronts 225/45/18

And rears 295/35/18

Mbren1979

It is difficult to answer as Porsche does not provide a lot of detail on how the viscous clutch for the front diff works. Finding specs on acceptable limits is very hard. Actual tire specs are different per tire manufacturer. but assuming we are talking a level playing field...... the stock front tires are aprox 1 inch larger in overall height which is about 3.8%.... Several people who have tried different sizes have had issues with PSM activating when it shouldn't be. Tons of other people have made comments about it damaging the Viscous Coupling. I personally run 225/40r18 295/30r18 on the street and 245/40r18 315/30r18 on the track which are 1.25 inches larger on front or 4.65%. This topic has been discussed a lot on the 996 Turbo forum (all were born awd). The general consensus on that forum is you must stay with these unless you plan on doing a 2wd swap.

Your rear tire size of 295/35/18 would put it 1 inch biased towards the rear. I personally would suspect it would be an issue not only with PSM/AWD but also would that tall tire even fit in the wheel well?

Just my .02

cansis

@LADO @SportsCarAdvisor What did you end up going with? I also have a 996 C4S and considered going with 225/45/18 and 295/35/18 as they are available in the Michelin Pilot Super Sports. Really bummed that Michelin only offers the old PS2s (and Cup 2s) in our factory sizes.

SportsCarAdvisor

Thanks for your insights helpful. I don't wan to tinker with the OE formula at this time. I ended up going with OE sizes.

SportsCarAdvisor

@Mbren1979 thank you.

Schnauzer

You can consider 235/40-18 and 285/35-18. This works fine for me. 275/35-18 would be even closer to ideal. Both choices fit on standard rims.