M3 S 2.3L Hatch Manual, 28.1MPG on my second tank of gas (two weeks old car), with about 50/50 highway/backroad.
Did a little of stop and go driving this time. On my previous tank of gas I got 30MPG with similar ratio highway/backroad but with no stop and go in traffic.
I'd think that 32MPG highway is achievable, especially once the enginer is broken-in