Borrowed from another forum:
Does this sound about right? I want to go squared and it looks like 19x9.5 +35 will be okay. I'm not tracking but shouldn't it apply to street as well? I'm trying to not use spacers if possible, factory alignment, and no fender rolling.
??
M3 Offsets:
Okay, so, now that you know what they are, how do you find the right one for the car? Well, originally with a measuring tape, shims, spacers, etc... but we're past all of that at this point. What works well on the e46 M3 is pretty well establish at this point.
So, here's how I'm going to do this: I'm going to list the best offset for a given size wheel (best meaning biggest tire fitting. You bling guys can make your own thread about stance, flush, etc), and the tire size you can fit on that. From there, if you wan to run a more aggressive wheel, you just decrease the tire size from there (aka, for every 5mm more aggressive you go, reduce one tire width (tire widths go in increments of 10mm, half goes on the inside, half on the outside, so every 10mm decrease in width brings it in 5mm on the outside)).
And I'm just going to focus on wider wheels with bigger tires, as smaller wheels/tires fit fine over a wide range of offsets.
Assume stock fenders, no rolling, no fender liner removal.
Front:
9": ET38. All tires 265 and smaller will fit, some 275s will fit.
9.5": ET35. All tires 265/35/18 and smilers will fit. All 275s will fit with some extra camber in the front. SOME 285s will fit-- this will vary from one tire model to the next.
10": There's no reason to go with a 10" front
Rear:
9.5": ET27 will fit all 275s and most 285s with the proper alignment
10": ET27 will fit all 285s and some 295s with the proper alignment
Special Case, track setup:
For the track, it's beneficial to run a square setup so you can rotate tires (extends tire life) and dial out understeer (matching front and rear tire sizes). The Optimal setup for this is 18 x 9.5 ET35, with a 10mm hubcentric spacer in the rear. This setup allows you to easily run 265 tires square, 275 square with some alignment tweaking, and 285 square if you work at it.
Take away: Clearly the offsets listed above are not the only offsets. What they are is the ideal offsets. Generally speaking, for every 5mm of variance of the above offsets, you need to decrease the width of the tire by 10mm.
NOTE WELL: There are lots of factors involved with tire fitment. Alignment, the specific fenders on your car, the model of tire you go with, etc. The above is generally true for the e46 M3, but there are exceptions. If you want to be guaranteed to have no fitment issues, don't push the tire sizes. The smaller you stay, the less issues you can run into!