Proposed Autocross tire & wheel setup:
I am considering the following autocross set up for my Cayman S, and would appreciate comments on this.

Front: 7X17” wheels with 235X40 Falken Azenis tires (tire diameter is 24.4”)
Rear: 9X17 wheels with 275X40 Falken Azenis tires (tire diameter is 25.7”)

This is for autocross and the shorter tires will improve the gearing a bit, and the Falken Azenis tires are an inexpensive proven street & autocross tire. I would prefer an 8X17’ wheel in the front but this proposal uses wheels I already have. Comments on this proposed set up will be appreciated.

I think that combination would exaggerate push. Porsche mounts 205s on 7" rims, so 225s on 7" rims is even a bit of a stretch, but work well on 7.5" rims. 235s really want at least 8" rims; 8.5" would be even better. With 8.5" rims you could also mount 245s, which would help the front/rear balance with the 275s in back (which would actually work better on 9.5" rims). I guess the real problem is the 7" rims you want to use. Those tire diameters also increase the front/rear diameter difference to 1.3". I don't know what the tire diameter sensors will say about that, if anything.

I agree with Jim... the narrow wheels on the 235/40s and 275/40s will also give you a whole lot of sidewall flex. Also, as Jim said, the 275s in the rear will only exacerbate the car's predisposition to understeer - 265s would be a better choice, considering the width of the rim you're using anyway. I would really recommend 18" wheels - while, as you mention, the gearing will improve, the smaller contact patch and sidewall flex you will have will more than negate any advantage you might get in gearing by using the 17" setup.

brad

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21-year PCA Member
PCA DE Instructor

I agree with these guys too. We run 245/275 RE01s on the 19" Carrera S wheels for autox and 245/275 RA1s on wider 18s on the track. I wanted as much front tire as possible to help the car rotate better.

What's wrong with 245/45/17 in the front and 275/40/17 in the back (falken Azenis).

I realize that the min rim width is 7.5 but many successful National AStock cars run 245's on thier stock 7" wheels.

I agree that 235's work better on a 7.5 or 8 inch rim. I have used this size Azenis 615 tire on 17x8 Fliks that I used on my '06 Jetta GLI. I have also used Azenis 275's on 17x9 rims and that is a perfect fit. The Azenis have stiff side walls and with the right air pressure (my guess is between 37 and 42 psi) they wont flex much.