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Old 02-07-2008, 05:29 AM   #1
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What happens if we put wider tires?

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?
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Old 02-07-2008, 05:57 AM   #2
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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.
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Old 02-07-2008, 06:14 AM   #3
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Quote:
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%.
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Old 02-07-2008, 07:48 AM   #4
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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.
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Old 02-07-2008, 07:55 AM   #5
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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.
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Old 02-07-2008, 08:23 AM   #6
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Quote:
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.
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Old 02-07-2008, 09:03 AM   #7
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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?
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Old 02-07-2008, 11:40 AM   #8
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You don't mention your rim widths...b ut, on my Boxster, which came with 18s...they were 7.5" wide front, 9" wide rear.

Fitting a 275/35-18 rear was no problem at all. Plenty of clearance

Fitting a 245 on the 7.5" front, was not recommended by many tire manufacturers. They wanted at least an 8" wide rim, but preferably at least an 8.5" rim.

As a result, I ran 225 front, 275 rear. On the track, and in ax events, the car did not have enough oversteer for my preference. They fit. They didn't rub. The car was very stable.

Depending on the offset up front, and if your car is lowered, you could have rubbing issues.
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Old 02-07-2008, 01:35 PM   #9
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I had a 245/40/18 on an 8x18 50mm wheel on my 01S. It worked fine. Agree with racer_d that a 7.5" wheel may be too small.
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Old 02-07-2008, 04:53 PM   #10
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Other than the issues already addressed above with rubbing, there is also the issue of tramlining. Wider tires will have more contact patch with the floor and with such small vehicles like our Boxsters you'll have an issue called tramline. Your tires will grip with the grooves on the road, especially freeways where the roads there have lines in them for rain purposes. The car will veer in the direction of the line in the road.

I'm running 18x10 with 275 width tires in the rear and there's definitely the feeling of tramline.
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Old 02-07-2008, 11:37 PM   #11
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yup, if you go wider in the back, go wider up front too..

wider only in the rear = more of a tendency to under steer.

wider only up front = more of a tendency to over steer.

for me, i went way wider up front, left the rear alone. and i will bring the rear toe back in check to get the back to behave.

(I autox a lot.)
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