speedo/odometer question

I have seen some fluxuations in my speedo/odometer and was wondering if any of you have witnesed or have hear of something similar. Got a 2000SX600r, The speedo seems to be ok and consistent with a buddies sled, (his speedo said 50, and mine said 50) but the mileage was off like 4 miles? So we took another sled out and same thing, Speedo was the same but mileage was different? Any hints/suggestions?