Jussi

My 993TT's backtyres are almost finished. They are stock sized 285/30x18 Pirelli Zero something.
I was calculating that with these tyres max.speed with 7000rpm is about 311km/h and if buying new tyres sized 285/35x18 P Zero Rossos max.speed would be 325km/h. This mod also lowers cruising speed rpms and makes ride more softer.
After that front tyres should be changed from 225/40 to 235/40 because of better diameter match.
Does anyone have this combo 285/35 + 235/40 and how they works?
Or should I just buy stock sized new tyres?

ca993twin

js-11,

I think your theory is fine, and if you match the fronts and the rear diameters, you should be OK. Be aware, however, that a "taller" tire (larger aspect ratio) will take some of the crispness out of the handling, as the car will roll over a bit more on the fatter sidewalls.

Jean

Excellent choice. The 993GT2 came with 285/35s as well. Helps you with the short gearing on our cars...and more comfort yes...+ more grip.

Be careful with the fitment, measure well the clearance. I know of someone here recently who had to return them

Jussi

Yes, maybe there is also little advantage in acceleration because 35 series tyre is thicker and softer and therefore has better grip.
Pirelli P zero have these both models, 285/30 and 285/35, and price is almost same, so maybe I'll make this cheap mod. The longer 6th gear would be 2000 euros and I still would have old tyres

Jean

In fact it is the taller sidewall that helps with cornering grip, more flex, more resistence against loosing grip, this is quite marginal however.. Remember that you will only benefit from the tires in top speed if your car can reach Rev. limiter in 6th.