I currently have the Staggered OEM 20" style 168 wheel set-up for my '04 X5 4.4i w/ sport package. They are mounted with the OE Michelin Diamaris (which have great handling, but are loud and don't last long) and I am not getting very good gas mileage (ave. 18) w/ mostly highway driving AND it is just about time to get new tires.

I am looking into taking off the staggered rears, putting ones like the fronts back there so that all of the wheels/tires are the same, so that I can rotate them. I would like to mount the wheels with Michelin Tour HP All-seasons from tirerack.com.

So my questions are:
1) Is this a good idea? (considering I will probably get better gas mileage and that front tires cost less than rear tires)
2) Will I need spacers for the back wheels if I eliminate the "stagger"? (because the off sets are different)