I believe the different gears are for sedans/wagons. The wagons generally have a relatively "heavy-duty" setup- larger swaybars, more rugged shocks, and larger-diameter tires... and I think this is where the different odo gears come in. A bigger tire needs less revs to go the same distance, so the slightly smaller odo gear compensates.

Or, maybe, I'm way off- never took physics in highschool...:)

I'd also suggest you hunt through the archives. This has come up before, and somebody's most likely come across with correct info, as opposed to my guess...