Tire size on Carrera III wheels - 18x8/18x10

[FoFoJoe]

I have Carrera III wheels on my 2001 Boxster, no spacer up front and 15mm on the rear. The rear fitment is spot on where I want it, the front could use a 4-8mm spacer but I don't rub at all front or rear so I'm happy for now.

Anyways, onto my question. I currently run the Bridgestone RE980AS front and rear. 225/40/18 up front, 265/40/18 rear. I don't drive the car much in the rain and never in the snow, but I do drive on sunny days in the cold, down to 35-40F. My rears are due for replacement, and I found a great deal on 275/35/18 replacements (same tire, RE980AS).

For comparison, the 275/35/18's are $320 brand new (2020 production) shipped. The same tire in 265/40/18 is $445 new, or $475 for the new RE980AS+. Looks like the + improvements are focused on snow driving, which the car will never do. I've got a Vic with snow tires for that.

From my measurements the 275 should fit in the rear without rubbing.

My question is, would the 225 front paired with the 275 rear cause too much understeer? I just use the car for spirited driving and the occasional commute to work (less than 3 miles). But my driving skills are pretty basic, so I am not pushing the limits of the car at all.

I'm planning on swapping to 235/40/18 fronts when the time comes to replace those, but the fronts have been wearing slowly so I have a year or two I would guess. Might replace them sooner if I find $400 burning a hole in my pocket lol.

pic for reference (also I just cleaned up the headlights last weekend after this picture, so excuse the yellowing)

[steved0x]

265/35/18 is the stock 18" rear tire for the 986 Boxster in 18". 275/35/18 should easily fit on a 10" wheel and would be a minor increase in tire diameter vs stock. 265/40/18 is a taller tire than stock, about an inch taller If you go down to 275/35/18 from that tire, your speedo will probably get more accurate, but if you are in the habit of going up to redline, you will get there at a different speed. This would likely only be a factor if you drive on track in spaces where you might top out in a gear right before a corner.

Another factor is going from 265/40/18 to 275/35/18 that it will leave more of a gap in your rear wheelwell. But if you are going up from 265/35/18 to 275/35/18 it will slightly fill that gap.

For street driving, in terms of widths, not canyon carving or driving near the limit, going to a 275 in the back and 225 front won't be that noticeable. If you are not sliding the car regularly now, I doubt you would feel a difference.

[FoFoJoe]

Appreciate the information. I figured my driving skills weren't advanced enough where I would notice the slight difference.

The only sliding I've done in a tiny bit here and there due to the current rear's being pretty bald.

For canyon carving, would you recommend a wider front tire? My front wheels are 18x8 so the 235 should fit no problem.

The cost difference between 225-40-18 and 235-40-18 is a wash.

[steved0x]

Originally Posted by FoFoJoe

Appreciate the information. I figured my driving skills weren't advanced enough where I would notice the slight difference.

The only sliding I've done in a tiny bit here and there due to the current rear's being pretty bald.

For canyon carving, would you recommend a wider front tire? My front wheels are 18x8 so the 235 should fit no problem.

The cost difference between 225-40-18 and 235-40-18 is a wash.

My experiences are on the 17" Boxster tires, and I really liked 225/45/17 front vs the stock 205/50/17. Both for looks and handling.

In 18"s, I don't have any experience running a 235mm wide front tire on te 986, but if it fits (and I am sure it does, 25.4" diameter on that size, plenty of room) I think I know some folks running that size on stock suspension) I bet it would both look better and provide a little better front grip.

[BrantyB]

For a point of reference I've autocrossed Nankang AR-1s 235/40/18 front and 275/35/18 rear on OZ 8/9 wheels and stock springs. The outer edge rubbed the fender with 5mm spacers in the rear, no rubbing without the spacers.

[FoFoJoe]

Do you know the offset on the rear wheels? I'm guessing the backspacing is very different between our sets.

And I'll take it the fronts didn't rub?

[BrantyB]

Originally Posted by FoFoJoe

Do you know the offset on the rear wheels? I'm guessing the backspacing is very different between our sets.

And I'll take it the fronts didn't rub?

9" rear OZ wheels have 43mm offset.

Fronts were fine and have a 50mm (not 57) offset.

[FoFoJoe]

For reference, the Carrera III's I run are ET57 up front, ET58 in the rear. I use a 15mm spacer in the rear, and will probably grab a 3-5mm spacer for the front.

8Jx18 ET57 (P/N 99736213600)
10Jx18 ET58 (P/N 99736214001)

I am on Eibach lowering springs, bringing me down about 1.5" if I had to guess. No rubbing as is.

From my math the 275's won't rub, but they will be here next week so we'll see.

[FoFoJoe]

Out of curiosity I ran the offset calculator to compare our set ups.

Our rear set up is identical when you aren't running the spacer (not accounting for wheel width, but with matched tires offset calculators read them as the same width.)
If I had to guess, your set up could be a tad thinner at the fender because your tire is a tiny bit more stretched compared to mine.

58mm stock -15mm spacer (43mm effective) on my wheels.
43mm stock, no spacer for yours.

With the spacer on your rear, you push to 38mm. So by that math, the extra couple mm should let the 275 tire fit without issue on my car.


Here are the fronts.



You're 7mm out compared to mine, with the same width wheel (8inch). I am currently running a 225 tire. With a 7mm spacer and a 235 tire, by these numbers I shouldn't rub. Especially considering you ran your setup for Auto-X, which in my mind would push the suspension to rub if the chance was there.

Main difference is our ride height. I don't know your suspension set up, but I am about 1.5inch lowered so maybe a 3-5mm spacer would be a better choice for me.

So your data was very helpful. Do you have any pictures of your Boxster with the auto-x wheel set up on by chance?

[BrantyB]

Yeah, your offsets should help your fitment and looks like you should be fine. The AR-1s have a pretty wide section width so that would also help your fitment based on the tires you're considering. I have a pretty aggressive alignment with 2.6deg Front (GT3 LCAs)and 2.4deg Rear negative camber. That's likely a wash versus your lowering springs.

On the front I did have a touch of rubbing with the wheel liner, but the liners tend to deform over time and can be re-molded with a heat gun.

From this pic you can get an idea as to how tight the rear was.