Switch to a narrower rear tire on my C4S?

Over the weekend I managed to pick up a huge screw in the rear left - thereby forcing me to switch to my snow wheels/tires a little earlier than expected (hope it doesn't warm up much). Fortunately the rears are pretty shot anyway (seems to be getting about 15k per pair of rears) so I will just replace them instead of getting it fixed. The stock rears for my car are 295/30/18 however, since the car tends to understeer by default, I have been toying with the idea of switching to 285/30/18 rears instead to see if this would improve the situation. Anyone know if that makes sense (it is financially good - about $30 per tire cheaper) and would do what I expect without doing other odd things? Thanks! Barry

Thought the stock size was 285 anyway?

C4S is 295. If you switch to 285, won't you be running different tires front and back? C4S rim is 11", would a 285 fit on 11"? I'd think it's okay as long as F&R tires are same and fitment to rim is not an issue.

Good catch on the tire size. A 285 will fit a 10"-11" rim. Should reduce the understeer a bit, but I would consult the C4S guys on the board to solicit advice.

I think it might wear even faster, I would stay with the stock parameters, it's only 30$, why screw around (potentially)?

I am not sure if you will notice the difference from 295 - 285 but with a 265 tire there is a huge difference. My rear summer tires 295 lasted 19,000 km and were almost down to the wear band (not bar that was totally gone) in the inside edge to middle treads. My new set of summer tires 295 have 8,000 km and are 2mm until it hits the wear bar on the inside edge. They are pretty much shot and I barely pushed them at all. I left psm on and didn't spin my tires. My rear winters have 13,000 km and they are almost down to the wear bar in the center tread. Inside tread has wear but not bad, outer edge tread is like new. I wish I could get 15k miles but I am only getting half that. If your getting 15k miles then your doing really well, though you must really never launch the car or do hard acceleration. The smallest tire that you could put on your stock rear rims is 265/35/18 These to me actually feel better then the 295 set but it is much easier to launch and spin the tires with the 265 set. I think the 295 are better if your tracking the car but the 265 and better for daily use. I think the 265 would wear more evenly and could last longer. I don't see any problem if you went down to a 285 set

You can't put 265's on a 11" rim , will pull the sidewall and bow in the tread, wear unevenly.

Yeah, 265 is too narrow for an 11" rim.

Well I disagree. The 265 I am running on are recommened for a 9.5 to 10.5 " rim. A diffenerence of .5 " is not a huge deal here. Additionally, some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. For example: BMW has combined 235/40R17 sized tires on 17x7.5" rims (which are 0.5" less than the narrowest 8.0" wide rim listed for the size) on certain M3 models; and Chevrolet has combined P255/50R16 sized tires on optional 16x9.5" rims (which are 0.5" wider than the 9.0" wide rim now listed for the size) on certain Corvette models. While these applications have received the approval of the vehicle and tire manufacturers, staying within the approved rim width range helps assure that the tire's internal stresses are within its design parameters. It will work and I find that the wear is not any worse then when running on 295s. There is no bow inwards. Thats for sure since the wear is now more in the middle of the tire tread as opposed to the inside edge. Which means to me it either bowed outwards or my tire pressure was higher and could be dropped down some.

Well, I'm going to have to disagree with you! The manufacturer recommends certain sizes for several reasons, including tread wear, and more importantly safety since a tire that's too small could come loose from a rim under high G loads. I'm sure it'll work. That's not the question. It's whether it's safe or not. A 265 is only 30 mm narrower than a 295, or 15 mm at each edge. If you notice a bow inwards now, it's likely that the sidewalls were pretty much perpendicular with a 295, which is optimal and how the tire is designed.

type o in my post "There is now bow inwards." meant to say there is no bow inwards. but back to the original topic running 285s are fine

Sounds like most people are impressed that I get as much as 15k per set of rears (and they are only down to the wear bars). As for not launching the car � I don�t spin the tires very much but I did do a full season of AutoX on them (well, 9 races if you include the one I did last year) with enough points to take 2nd place in my class. The cost saving of $60 is not that big a deal � it would have been just a nice bonus if people came back and said that they had done the same thing and felt the car was better balanced. As I have not heard this I will just stick with the stock sizes. As for tire sizes � my snow tires are 265s and I don�t have any problems with them. Then again I am not exactly pushing the g-force envelope with those babies on either. Thanks again for the advice! Barry

Your 265 snow tires are mounted on what width rim?

When I switched to pirelli corsas for track (R-compounds) i went with 285's for the rears....the R's are a world of difference from street tires so could not say whether the size made any difference with under steer. You are obviously theoretically correct...but there is absolutely no problems in swithching to 285. Another guy on the track was running 265 R's and loved it...he did leave early as the tires worn down, but he may have been using it for a few events by then...but he did tell me that he absolutely loved it. He was driving a C4S as well.

I could not put my winter 265's on my stock rims and had to buy 10" rear rims for them. I am amazed how little the Perelli Winter Sports do not wear, harder rubber I guess. I was thinking about putting on 305's on my summer rims when I wear out the rear 295's because that is what my model year Turbo uses with the same body. Any thoughts? Will

I have the Wintersport M2 265s on the stock rims which was done after much discussion with various people on this board, at the local Porsche dealer, at the Porsche club, the nice man at Tirerack and my local tire dealer. None of them said that a 265 would be a problem (but they did say that I could not go any smaller than that). If you go to Tirerack now and select winter tire packages for a C4S it will display 2 sets with rears that are 265. This is new this year, last year they told you to call (which is what I did). Barry