I use GPS for speed validation and also to confirm odometer readings.
Typically at fill up I zero out the trip meter on the Speedo and zero out the GPS distance/trip meter. After 100 or so miles (or more) I compare the two. This gives me the error correction % for calculating true distance traveled for accurate mpg calcs.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com