What happens if we put wider tires?

[motoadve]

My Boxster came with Boxster S 18" wheels
235/40/18 fronts and 265/40/18 rears

What happens (Pros and cons) if I put 245/40/18 fronts and 275/40/18 rears?

Will the PSM be affected? car will have worse handling or better?
Will the tires fit the rims? Any risks?

[drburton]

I went to a wider rear tire and left the front the same. Going wider in the front could cause some rubbing issues if you have the wheel turned all the way (for example in a parking lot). In the rear you should be fine with a 275. I don't think there will be too much of a change in performance, but will improve the appearance. Maybe a bit more grip at launch.

Be careful with how the width impacts the circumference of the tire. If you go wider you generally need to have a shorter sidewall. So in the rear, you should probably go with a 275/35/18.

[blue2000s]

Originally Posted by drburton

Be careful with how the width impacts the circumference of the tire. If you go wider you generally need to have a shorter sidewall. So in the rear, you should probably go with a 275/35/18.

Actually, you want to keep the same sidewall height, independent of tire width. In some cases that means going to a smaller aspect ratio.

In the case of a change from a 265/40-18 (~12.54" radius), the closest match is a 275/40-18 (~12.68" radius) which is a 1.1 % increase in rolling radius. A 275/35-18 would get a ~12.22" rolling radius, or a reduction of 2.55%.

[pompous]

That may be the case, but I also went w/the 275/40/18 on the rear and regret it. I wish I would have gotten 275/35/18 instead, b/c each time I hit a bump w/much speed at all it makes this horrendous rubbing noise. I've been told its pretty easy to fix it, but never got to it and now I can't remember how.

[Kirk]

Originally Posted by pompous

That may be the case, but I also went w/the 275/40/18 on the rear and regret it. I wish I would have gotten 275/35/18 instead, b/c each time I hit a bump w/much speed at all it makes this horrendous rubbing noise. I've been told its pretty easy to fix it, but never got to it and now I can't remember how.

Roll the fenders in the rear or have an alignment done for more negative camber. I just put up a post recently about me rolling the fenders on my car to fit 275s in the back. Seems to have worked well.

[blue2000s]

Originally Posted by pompous

That may be the case, but I also went w/the 275/40/18 on the rear and regret it. I wish I would have gotten 275/35/18 instead, b/c each time I hit a bump w/much speed at all it makes this horrendous rubbing noise. I've been told its pretty easy to fix it, but never got to it and now I can't remember how.

You'd have the same problem with a 35 aspect ratio tire if you're rubbing on the fender.

You've either got to change your offset (and make sure you're not touching any suspension components) or roll the fender, as Kirk says.

[humara]

just a guess here, and i'm sure people will chime in, but increasing the width in the rears will increase grip in the rear a bit. unchanged grip in front would mean....


more understeer



is this what you want to accomplish?