: : The L6 TRX's had an overall diameter of 632mm.
: : (The M6 actually had an OD of 631mm)

: : The wheel you are suggesting has an OD of:
: : 15*25.4 + 2*.6*225 = 651mm

: : The setup you are being tempted by is about
: : 20mm larger. That's 3/4 of an inch! I would
: : not buy it.

: : The M6 rims were over 16 inches. Your car
: : had a rim under 16 inches. So it makes sense
: : to pick a rim size in the middle; i.e., 16".

: : Again the 225/50 ZR 16 diameter is exactly the
: : same for all intents and purposes (631.4mm).
: : I actually run a 245/45 ZR 16 in the rear in the
: : summer.

: : Now you have to worry about offsets. Your rim
: : width was 165mm = 6.5 inches with ~21mm offset.
: : You don't specify rim width and offset, and
: : don't bother until you have 16" rim picked out.

: How are you figuring that out. Would a 215/60/15 be a closer match? I'd like to stay with the 15" rim because of the price I can get on them.

Yes, after thinking about it, a 215/60/15 sounds
like an excellent choice, especially in the front.
It would have an overall diameter of:
15*25.4 + 2*.6*215 = 639mm (You should see how
the calculation works. 1" = 25.4mm, which don't
need for metric TRX wheel diameters. Add two
sidewalls for top and bottom.)

The problem with wider tires in front is that
it often messes up the steering geometry. In
the rear there is really no downside. I would
go with 225/55/15 in the rear. That would give
you slightly larger narrower tires in front.
And it would give you slightly smaller wider
tires in the rear. That is how new BMW's are
configured from the factory, due to the improved
tire technology.

Just make sure the offset is very close in front
(or that you can get there with spacers).