225 rear tyres for a 3.2, what wheels?
 

I'm thinking of putting 225 tyres for more rear end fluidity/oversteer on the track, leaving the 205 on the fronts as stock, rather than going for wider fronts. What weel witdh would be best for it - 7 or 8.5 inches (17 inch) , or something in between ? Anyone has any experience with this setup, which would be quite similar to oringial 205 & 225 setup on 16inch wheels?

This is a first! I have never seen a driver looking for less contact patch in track tires. If you are running a DE, balance is more important than lap times and your brakes will last a lot longer.

225 on the rear will help balance the car at the expense of a little lateral grip and stopping power. Dunlop says you can mount a 225 on your stock 8.5 wheel: http://www.tirerack.com/tires/tires.jsp?tireMake=Dunlop&tireModel=Direzza+Sport+ Z1+Star+Spec&partnum=245WR7Z1SS&vehicleSearch=fals e&fromCompare1=yes

Have fun!

I'm not looking for lap times or ultimate grip - that's why. I'm sure you can fit a 225 on 8.5 rim - Porsche even states this for the winter tyres and people on here have fitted a 225 on the 7 rim although for the fronts. But which is better rim witdth for the tyres ?

You will need to check the specs for the exact tire you plan to run. For Dunlop Z1 Star Spec the ideal rim is 7.5 with a possible range of 7-8.5. Wider rims give you more sidewall stability, narrower rims give you less unsprung weight. Every choice is a compromise depending on your desired outcome.
http://www.tirerack.com/tires/tiretech/tirespecskey.jsp#measrimwidth

that setup will look goofy as hell with such small tires. good luck with it all

Topless, thanks for the valuable contribution. I'm willing to experiment (on the track). Having 225s I can always put them on the front I they don't feel good on the rears and leave there the 255s - just one more option ;)

Honestly, its going to be a mixed bag, one way or another, as long as the rim width falls into the recommended range.

Like said earlier, the smaller size will lower unsprung weight, and make the throttle feel a bit more responsive, if not lively. The sidewalls will flex more, as the car gets thrown around, and might not feel planted enough.

The widest choice will roll onto the outer edges of the tread, and chew them up pretty quick, giving you a tire that goes away much faster than expected, and overheats, but gives you a stronger sidewall, that doesn't flex much, and allows you to predict when that breaks away more consistently.

The preferred rim size will give you a good balance between the two.

The 225/45-17 is the same diameter as the 255/40-17, as I'm sure you know.
If I were buying winter tires for the Boxster, that's what I would put on it.
I don't know how well you're going to do with getting the power down in a DE with that size, with one tire unloaded and spinning, and the tire fighting to keep control of the car, and heating up rapidly.

Would be interesting, to say the least.

If money's not an object, go with the recommended rim size.
Better to balance everything, than to wonder what if later.

BC.

Thanks again.
My rear camber is actually nearly 3 deg and front just 1.5deg, but that may need changing at the later stage. I'm sure the 225s on the rears are not too small in general for the car of this power & weight, that's why I'm willing to experiment. Hope to post my impressions next month provided I get the chance to try different sizes when I do the track.