Lambeau,
Here's a couple points to ponder: I recall reading a few posts that
were adamant about the Odo not being "off ~6" like the speedo is.

So, you may not realize any "less" mileage via the odo...

2nd point on the arith., my version of gozintas:

-Your big summer 20s were/are about 4 mph less
than stock 20 tires, or ~6% less than stock in
terms of tire revs per mile.
-Your big winter tars are about ~2 mph less than
your current big summer tires, and about ~4% less revs per mile.

Put that all together with the very optimistic BMW speedos and
I think you can drive about 80 and still be under the radar, lol!
GL,mD