My R1200GS' odometer is about 1.5% pessimistic. That is, when the GPS says I went 100 miles (using a straight stretch of I-90 in Washington, USA) the odometer says 98.5 miles. Same story doing the mileage check for the Utah 1088 -- mine was about the only bike with an pessimistic odometer. Most are optimistic (like my old R1100GS, which was 5% optimistic (it said I'd gone 105 miles when I'd really only gone 100).
That's with the OEM Trailwings, BTW. Now I have BT020s on it.