JPS

I have 7" front, 993. I've been running 235/45/17s on them for several sets. Toyo RA/1 specs say those tires can only go down to a 7.5" rim, and actually like an 8" better. Is it really that big of a deal on the 1/2 inch difference to the 7" I have? I've been running them for probably 4 sets RA/1s without a problem, but when I called Toyo, they sort of freaked me out.

986

I ran many sets of 235/45/17 on 7" (Boxster S fronts) with no issue. It was recommended to me by my tire shop (a well known shop for race tires including Toyos).

racer

I think the issue here has to do with maximizing the sidewall geometry and strength. The wider wheel allows the sidewall to sit "straighter" and arguable is both stronger and more predictable under heavy corrnering forces.

mark kibort

no big problem there. It pinches the side wall in a little, and only will make the tire act a little different compared to a 235 on a 7.5 or 8" rim. 7.5 is more optimal though, and the tire might wear better as well as get more active contact area, with the larger rim. you might be able to go lower pressure to counteract the downside effects.

WHB Porsche

Let's say 215 is the ideal width for your rim (I'm not sure, but that sounds about right). If you go down to a 215 from a 235, you're losing 8% contact patch (assuming all the tread is being put down with a 7' wheel) but probably improving the "geometry" of the sidewall by more than 8%. It's anybody's guess whether or not you'll be "better" off with a properly-specced tire, but sidewall geometry is important for the feel and stiffness of a tire.

Bryan Watts

Contact patch size doesn't really change when you change tire widths...only the shape of the patch.

J richard

I don't think you're out of the envelope at all, I've run 245 s on that same rim and a lot of the 964 guys do it's pretty common. The thing you loose is the sidewall stiffness that allows the wheel to move around inside the tire. Its not the lateral movement (in cross section) but looking at the tire /wheel from above, with a "soft" sidewall the wheel will turn into a corner but the tread with it's friction against the track will resist and the wheel will twist and squirm within the tire, that will give you a slower and less crisp turn in and a less efficient/consistent contact patch, so you have to play with pressures like mark said which WILL reduce your contact patch like Bryan said...that's the compromise IMHO...I've come to prefer a slightly narrower tire up front,
better feel...but depends on your setup...

analogmike

Why not buy some 8" rims? I bet you would be faster on 8" rims and 225s than 7" rims with your 235s.

I find 225s are the widest tires for ultimate grip on 8s and I need 9s for 245s.

JPS

Thx. In answer to some of the Qs, in the Toyos in 17's you have 235 or 205. 205 would understeer (used to have them as it is OE spec). IMHO 225 would be great, but not made in that tire.

As far as getting new rims, just haven't gotten to it on "the list". I'd probably switch to 18s and go 8.5 / 10.5, but haven't found what I want yet.

fatbillybob

Is it "safe" to run big tire on little wheel with its unsupported sidewall. I would think the performance would decrease but I wonder if you could blow a bead under heavy cornering? If you decreased pressure to improve the contact patch from the small wheel pinch that would make the sidewalls even less stiff. Somehow this does not sound like a good idea even if you can physically do it. Is the safety issue why toyo was all in your underwear?

Geoffrey

The issue with running the wide tires on the narrow rim is that it changes the sidewall geometry as mentioned. The rim will tend to float inside the tire and you'll lose feel in the end of the car. As an example, in the guide to slicks from Michelin, they say to run their tires on the maximum width allowed by the tire due to the suspension of the sidewall. As an example, I run a 240 slick on a 10" front rim. The turn in is much more crisp than when run on an 8.5" rim. Personally, I don't think you'll gain more than you'll lose by running a wider tire on a rim it was not designed to run on.

machina

Grassroots Motorsports did a test using very wide and very narrow tires on the same rim. Cant remember the sizes exactly but it was something like a 285 on a 8" rim vs a 235 on the same 8" rim.

Result was that the narrow tire beat the wider tire on a timed auto-x course and the wider tire killed turn in, overall feel, etc.

I always run the widest rim possible.

sig_a

1/2" wheel width translates to .20" overall tire width. Yoko, Toyo 888, Nitto NT01 offer 225/45/17 mounting to 7". 225/45/17 is 8.9" width vs 235/45/17 at 9.1".