Previously, I had only calculated my gas mileage one time; however, that was via the bike odometer and that tank was 49.3 MPG. For my last tank, I kept 2 mileage records. The bike odometer and the GPS. The total miles differed by 10%. The resulting MPG (GPS) was 59.6, while the bike odometer calculation was 54.4.
For those of you relying on the bike odometer, I think it would be safe to say that you could add 10% to your calculated MPG and get a more accurate estimate of your MPG.