Speedo differences?

[langow]

A buddy of mine has a short track Viper and I've got an LTX. We rode together this weekend and when my speedo is reading 87, his is reading 80. Mine is dead on to GPS at that speed. Also, we both set our trip odometers at the same time in the morning. At the end of the day mine read 190.3, his read 179.0

WTH? Anyone else notice anything similar? What would cause differences so big?