i am currently running cup 1rims with 225 in front and 255 in the rear. I am considering getting some custom wheels made for me. I would like the rims to be as flush with the fenders as possible without having any rubbing issues. Also I want them to weigh way less than the aluminum cup 1s. My fenders are rolled from a previous owner. For the fronts I was going to go 17x8 52et. That seems like a fairly safe size from the reading I have done on the forums.

Here is is my actual question. For the rears I was thinking of having a 17x8.5 wheel. The 8.5 inch wheel should be a bit lighter than a 9 or 9.5 inch and the 255 tire still fits. My issue is like I said I want to go flush with the fender without rubbing and I cannot find much info on 8.5inch rear wheels and offsets used. The RS Cup car is a 9” with a 55et. If you want to leave the outside lip in the same position on an 8.5 inch wheel it would be approx equivilant to an 8.5 49et. I’m pretty sure those are not flush with the fender. I saw that people are running 9.5 et68 with 21mm spacers which is equal to 9.5 et47. If I use an approx equivilant to keep the same position of the outter lip that would be an 8.5 et35. Can anyone confirm if I am correct and if that would work. My other concern is maybe the wheel won’t fit over the calipers. Maybe that’s why the high offset wheel with the 21mm spacer is used on the 9.5 wide wheel. Is there anything else that I should be concerned about?

Ok thank you. I thought that the tire should be the maximum size for the rim to get max performance. I thought as the tire was stretched the performance went down. I guess I had it backwards. What is the best width with a 255. A 9” or 9.5”?

Why is the tire better on a wider rim? Is it because there is less flex in the sidewall? I assume the contact patch of the tire always remains the same regardless of the size of the rim. Or does that change marginally as well depending on rim width when the tire is stretched?

Perfect thank you for the info. Last question. When you say a 255 can use a 10” et~51 +/- . Is 51 the ideal offset in that width? The +/- is that beacause different manufacturer tires fit slightly different so you may be able to have slightly higher or lower offset on a 10” rim.?

Here is my rear BBS E26 wheel 17x10 +47 with a 5mm spacer making the effective offset +42. Tires are 255/40/17 re71r. -2 deg of rear camber. Rear fender lips are ground down for extra clearance.

PERFECTION!

what are are you running for caster?

I am not really concerned about it rubbing the upper fenders. I’m more concerned it will rub the inner fender liner. I have the corner of the bumper/fender liner trimmed to allow max clearance. But I don’t want to have to back the caster off below rs spec to accommodate a wheel.