RonCT

OK, I have several years of experience with R-comps in the most advanced run groups. I had a great experience with Michelin Pilot Sport Cups on my M3, and had a less-than-perfect experience with RA1s on my 997S simply because I was "cheap" and didn't have them shaved (wanting to get more life out of the tires). Now that I have the 92 C2, I'm trying to figure out the optimal combination of R-comp tires and fortunately (or unfortunately for making a decision), there are plenty of options.

All have said to stick with an 18" format, which I understand and agree with. But then here's the follow-up question (about 50/50 opinion on this)...

225 or 235 up front - many tire companies have both. Rolling diameter is 25" on the 225, but closer to 25.5" with the 235.

255, 265, or 275 at the rear. Not all tires have all three choices, but here are a couple of the typicals:

PSC in 225 or 235 up front and 265 at the rear.
Toyo RA1 in 225 or 245 up front and 275 at the rear.
Toyo R888 in 225 or 235 up front and 255 or 265 at the rear.
Nitto in any combination.

Some have advised that 225/265 works very well, but is there a better balance to say 235/265 - a little more grip up front and thus a little bit less understeer. Perhaps the 225 front will heat up sooner or perhaps have better temperatures across the whole tire and therefore be the superior choice to the wider front?

Anyway, here are the specifications - opinions very welcome:

Michelin PSC
225/40-18, 25.1", 20 lbs, 9.2" tread
235/40-18, 25.4", 22 lbs, 9.5"
265/35-18, 25.2", 24 lbs, 10.9"

Toyo R888
225/40-18, 25.1", 23 lbs, 8.8" tread
235/40-18, 25.5", 25 lbs, 9.6"
255/35-18, 25.1", 26 lbs, 10.2"
265/35-18, 25.4", 27 lbs, 10.8"

Toyo RA1
225/40-18, 25.1", 23 lbs, 9.2" tread
245/40-18, 25.59", 26 lbs, 9.65"
275/35-18, 25.55", 30 lbs, 10.94"

Nitto NT01
225/40-18, 25.08", 9.06" tread (no weights given)
235/40-18, 25.39", 9.49"
245/40-18, 25.55", 9.72"
275/35-18, 25.55", 10.91"

Michelin is the low-weight winner and has the correct diameters. R888 also has the diameters, but I'm sure some of the Toyo products (including the Nitto) can be shaved to get closer to stock rolling diameters.

Bill Verburg

A difference of .3" is pretty small(225 vs 235)

.3/2 = .15" ride height increase.

RonCT

Thanks Bill - one of the things I've heard is if diameters get too large, you can have rubbing issues on cornering, especially up front. So if a tire is 25.5" and is shaved, then I would think that brings it closer to the 25" OEM diameter. What is your thought on "too much" width up front (235-245 vs. 225)? I know everything is a balancing act - I wouldn't want 225 front and 275 rear for example. But 235/265 PSC vs. 225 front? R888 in 225/255 vs. 235/265 or 225/265?

Steven C.

FYI I have been told by tire experts that shaving them actually makes them last longer. Less tread squirm = less heat. I shave my RA1's and thay will last 6 to 7 weekends for me.

Bill Verburg

i'm no expert on 964 firments but I see a lot of guys w/ 245s

whether it's worth doing I don't know, you can dial under/over steer in w/ adjustable sways and you can make the front more or less reposive w/ suspension settings in addition to tires

I use 225/265 on my 993 w/ adjustable shocks and sways, 235/275 on my C3 w/o adjustable anything. Both are reasonable setups. It was a lot of work to get the 235s not to clip the fenders.

TR6

R888's don't come in 275? Seems like they would since the RA1's did. I run RA1's at 225 front and 275 rear and it seems to handle fine. Not too much understeer. But I'm certainly not an advanced driver by any means. I didn't want to risk 245's up front for fear of rubbing. The ideal combination seems like it would be 235/275 but RA1's don't come in 235 (at least I haven't found them). And if what you say is true, R888's come in 235 but don't come in 275.

For what its worth, in street tires, I run 225/265.

Manny Alban

I'm going back to MPSC for 08. I ran 225/265 and later 235/265. I'm going back to the 225/265. I couldn't get enough heat into the 235. I also asked the Michelin guys at Sebring about shaving the MSPC and they said it best not to shave.

RonCT

Thanks Manny - when I ran PSC I didn't shave them either - they were great from the start once some heat got into them.

Cupcar#12

just for point of reference the orignal 1990 Cup ran 245/265mm section slicks
i would go for the 235/265 combo depening on agressive you are with the front - if your not the 225 would be the choice since it will heat up sooner.
this also depends on your choice of shocks, etc.

RonCT

Thanks - I don't have the figures on alignment handy, but it's not full-out race, but rather DE / aggressive street. I have a JIC Competition suspension running 564 lbs front and 674 lbs rear. Ride height is Euro RS (-2"). Sways are RS 5 way front, 3 way rear.

GP White

Not to confuse the issue even more, a Michelin engineer once told me the MPSC works better when the tire is on a wider rim for its size. In particular, the 225-40-18 MPSC works on rims between 7.5" - 9", while the 235-40-18 is for 8"-9.5" rims. Since I was using 8" front rims the Michelin engineer recommended the 225 over the 235. If you are using 8.5" front wheels, you may want to use the 235, but the 225 may still provide better grip.

Bill Verburg

Might want to think about the ride height. The best info will come from the guys racing Cups and their feeling(from what I've been able to gather) seems to be closer to RS. They aren't allowed to run EVO uprights which may be coloring their thoughts a bit.

RonCT

Thanks Bill, my shop maintains 6 964 US Cup cars and they "tweaked" the ride height, alignment, etc. for me and how I plan to use the car (DE, Black Run Group).

Geoffrey

Slicks are measured differently, so that isn't necessarily a good point of reference.

It is true that the Michelins should be mounted on the widest rim possible, that is how they have designed their tires to work, both the PSC and the Slicks.

I have never really liked the PSC tire and think there is are better alternatives for a DE/dual purpose tire. If you can get the PSC in its right operating range with enough camber, then they do work, but as a dual purpose tire, there are too many compromises that need to be made and that renders the tire ineffective.