speed ohmeter is fast

[kwplot34]

99 Fleetwood tioga ford E350, V10. Is it normal for the speed/o to read fast on motor homes. Ours compared to the GPS reads 5 MPH fast. At 70 MPH the GPS says 65MPH. The GPS is accurate, have had it for several years and compared it with our other 4 vehicles and they read the same MPH.

[Cobra21]

If the tires are smaller than factory spec. it will read fast. Check if they
are the correct size?

[Triker33]

How about the odometer difference compared to 10 miles of mile markers?