Over Drive is really over rated. In auto mode it will advance the CVT to the same ratio but it will not stay in that ratio as you move the throttle or hit a hill. Putting it in OD locks it at that ratio so if you are on fairly flat ground it will not drop the ratios and you MAY get a bit better gas mileage. I can stay in OD even in some of our mountain passes and I get 3-5 better MPG. Thats about 15 to 20 more miles per tank and on a long trip it matters a lot.
On the 02-04 non-OD 650's if I were cruzing in auto on real flat ground and was very steady on the throttle it would drop the RPM's 100 to 200 lower than if I locked it in 5th. So we were getting OD but could not lock it in. But I averaged 54 MPG even without a true OD.
I live in the mountains and ride my 650 pretty hard with speeds limited to 40-50 due to the curvy roads. I get the best mileage by leaving it in D and letting the bike do the work. In general running around, I get in the high 40s and traveling on longer trips, I get in the low 50s (52-54).It's all in how you drive and what the terrain is like. Here in FL I have flat ground and I hammer neither the throttle nor the brake, leaving lots of room around myself to escape on a moment's notice. I regularly get 53mpg while commuting on my 650.