I read through the sticky, but just wanted to confirm, for 15x8 rears I want:
But which offset would be needed? Any tolerance in backsapce, or does it need to be exactly 5.5?

Also, will a 275 60 15 BF Comp T/A DR fit on the rear, stock, clearance issues, got some for free, just seeing if I can run em'?

 

The 275/60-15 is a 28" tire. It may fit, but most likely you will have problems with it hitting the edges of the wheel openings at the front (sheet metal on the 1/4 panel) and rear (leading edge of the bumper cover). I ran some Hoosier 28" QTP's and had to relocate the axle with adjustable LCA's to keep the front edge of the tread from hitting the 1/4-panel. With radials, you won't have the problem of diameter growth with high speeds, so it might just fit.

And of course, a tire that big will make you 3.42 gears feel like you swapped down to a 3.14 (if there was such a gear). Big tires will kill performance, unless you compensate with some big gears, but even then the weight and diameter will really rob rear wheel HP.

You might want to think about the 275/50-15 instead.

Backspace and offset are mathematically related. If you get the right offset, you will get the right backspace and vice-a-versa.

15x8 with the 5.5" BS puts the outer edge of the tire just barely under the edge of the fender. Go less than 5.5" BS and you will have the tire creeping outside the fender. Look at the picture in the FAQ. That's a 15x10 7.5" BS, but your 15x8 5.5" BS will sit in exactly the same place. You could go to a 6.5" BS and move the tire 1" deeper in the wheel wells.

 

Thanks for the indepth post, appreciate it. So my best chance for it to fit would be where the tire sits @ 5.5" backspacing? Only reason I'm trying, I just got some 275 60 15 DRs for free, from a friend who decided not to run them, and went with something bigger, so I was trying to make em' work if possible, otherwise I'd just go for something like 275 50 15 like you were saying.....

 

The 5.5" BS will put the outside edge of the tires in the same place as these:



That will leave you with almost 2" clearance between the inside edge of the tire and the inner fender liner. So you can move the wheel/tire into the wheel well by that amount (depending on how you want it to look). A 6.5" BS would move the tire 1" deeper into the wheel well. But I doubt anyone makes a 15x8" wheel with a 6.5" BS. Its hard enough to find the 5.5" BS. You simply do not want less than 5.5" BS. or the tire will be outside the fender.

The 275/60 MIGHT work. It will not have the growth problems that you get with true slicks. My 28" slicks didn't start to hit the fenders until trap speeds went over 125MPH.

 

We'll I mean, actually its just temporary, so I dont really care how far it sticks out there, cosmetically speaking, as long as its not utterly rediculous. I've noticed as deep as you go in the BS, like 5.5 wheels seem to get way more expensive. The car only sees the track about 3-4 time per year, and I'd just swap these on for the drive there(50mi) and then back to my street tires/wheels when I get home. So is the reason for not having less than 5.5 simply cosmetic? Or will the chances of it rubbing increase the further the center of the tire gets to the lip???

 

Some class rules will not allow the tires to extend outside the body. Depends where you're racing, what class you're racing, and whether anyone in "tech" checks/cares.

Obviously, if your suspension squats at all on launch, you will put the fender lip into the sidewall or the tread.

 

Alright, I'll just have to scrap that idea, now I'm just going to look for a more reasonable size tire, only question is, I still want to run 15" wheels, what tire size would you recommend as to not kill the gear ratio? or maybe even help it, 3.42---->3.50 with a slightly smaller overall diameter? These won't be used on the street at all(except driving to the track) so they can be like any form of DR/Slick - and I'm basically stock, and an a4 so I don't really need to big of a tire.....just wanna keep it cheap, as they won't be used that much, thanks for all the help so far Will I still need to run the 5.5 BS with a 275 50 15 or anything around there?