Hi Mike,
Perhaps I can shed a little light on your dilemna. I run a '99 Z3 2.8 Coupe
which came stock with 16x7 rims all around, but which had factory option
17x7.5F/17x8.5Rs, identical to the staggered E36 M3 setup. When I first
went to Kumhos, I bought a set of staggered E36 M3 wheels and mounted
225/45/17F and 245/45/17R Kumhos on them. If you look at the data sheet on
TireRack, the claimed outside diameter for the 245/45s is only 0.1" larger
than the 225/45. So I bought them thinking it wouldn't be a big deal, since
they were supposedly sized closer to a 245/40. Well, in reality they are a
good 0.6" larger in diameter than the 225/45s, so don't go off their
published specs!
Anyway, I wasn't very happy with this setup as the rears were too big, which
had four major disadvantages:
1) taller gearing
2) extremely heavy, even on the forged E36 LTW rim (50lbs total on a 22lb
rim = ~28-29lbs of tire, ouch!)
3) even harder to fit inside my car for transport (yes, 4 tires do barely
fit in the Coupe)
4) wider rear tires useless on a car that understeers like mad to begin with
So there were two options, go 225/45 front and 255/40 rear, or 225 all
around. I wasn't too crazy about the first option, because it made problems
2, 3, and 4 even worse. So I was all set to go with 225/45/17s all around,
which, as crazy as it seems, would probably work really well.
Anyway, I didn't end up doing that in the end because of a very generous
fellow competitor who shall remain nameless. Turns out said person, who I
shall refer to as "Mr. Generosity" runs his Kumhos on the street and doesn't
want to risk running them too far past the wear bars. These nice shaved
tires happened to be 245/45/16s, which fit my old 16x7 rims. So thanks to
"Mr. Generosity", I am back to running nice fat tires same size around and
they are small enough to fit in my car with tons of space to spare! So I
gave what was left of my 245/45/17 Kumhos to James.
Anyway, I'm getting a bit off topic here, but my point is that I don't think
it's such a bad idea to try 225s all around. I can't be sure that they will
fit the wide 8.5" rear rim, as the Kumho published specs are slightly hokey.
For example, my 245/45/16 V700 casing is about as wide as my 245/45/17
Bridgestone RE730 street tire, but the tread is wider on the Kumho. The
225s are about 0.5 inches narrower as advertised. Our BMWs are probably
very neutral at track speeds, but for autocross they have a bit of low speed
understeer. If you can't get a wider front tire on there I don't see what
good a wider rear will do. Also, if you plan on running some of the SFR
events at Golden Gate Fields, the 225s might even work better because that
surface doesn't warm the tires up much.
The disadvantage is that the M3 has more power and torque than my 2.8, and
so it might be harder to get power down on corner exit. But if your 245s
aren't even getting warm, then that might not be the limiting factor. Hard
to say, but you might want to try it. Good luck!
Peter
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