From my friend at the Tirerack (I forgot that there is a specific tire section here at 1Addicts):
Our guy Gill moderates the wheel and tire forum on the site, and has owned a 135, so he gets it, and would be a good place to point people as I am not in sales here at the Rack. I’ve copied Gill on this so he is aware of our conversation.
We’ve had a 1M here to physically measure, and our fitment engineer is a BMWCCA track guy, so he has great personal interest in the car.
Fitment front and rear is a little tight. And how tight is too much may depend on personal preference, and how much self-clearance you feel you can get away with in SCCA stock class (technically, none). The ideal track set up with Hoosiers is 275/35-18 front and rear on 18x10 wheels. Offset is critical, as there are clearance issues front on the inside as well as at the fender lip. Also, for those cruising info on the web care should be taken when looking at street tire fitment info versus what a Hoosier in the same or similar size will be like. Overall diameter also begins to play a role in the fitment, especially in the front.
275/35-18 works great front and rear on the proper sized and offset wheels. But as everyone is finding, the 19” Hoosier options for the front fall on either side of ideal. 235/35 is too narrow for sure, and the wider and taller than OE 265/35-19 may have clearance issues due to diameter. Anything wider on stock offset front wheels may need a small spacer to push the tire away from the strut, as inboard clearance is tight.
Personally I would be tempted to try the 265/35-19 on OE wheels in the front, being prepared for some rubbing and the need for spacers and/or a tire size change to a narrower/shorter size. This one is close enough that it will need to be fitted and driven to verify that it won’t rub lock to lock with the dynamics of suspension travel, etc. I’d match that with 265/35-19 on the rear for a square fitment. I think this car will have plenty of low- to mid-speed understeer inherent in the chassis, so going wider on the rear will only increase that. And let’s face it, lots of autocross time is spent transitioning and in the corner. Less time is spent powering away from the corner. With limited negative camber in the front and very low profile sizes my guess is this car is going to eat front tires if care is not taken. Just look at what happened for most people that tried 135s. The shoulder of the front tires corded quickly. You can probably get away with 295/30-19 on the OEM wheels in the rear, but again I don’t think it’s the ticket.
There just aren’t that many cars that will use modest width 19” R-compound tires, so I would not hold out for Hoosier to tool up anytime soon. And as awesome as it will be for you guys to race this car, there aren’t all that many 1M cars that will be autocrossed in stock class, so…
Here is a matrix of some Hoosier dimensions for comparison:
________________________________ section width when
________________________________ mounted on X" wheel
Hoosier A6_______OD (in.)__________9”______10”
275/35R18_______25.5_____________10.5____10.9
235/35R19_______25.6_____________10.2____10.6
265/30R19_______26.1_____________11.1____11.5
OEM
245/35R19_______25.7_____________10.0
265/35R19________________________________10.9
Thanks, and keep me posted as to what you try.
Enjoy the new ride!