Speedometer not accurate?

[tony_fury]

Anyone notice a difference between the car speedometer and GPS speed?

Since the installation of my Kenwood with Garmin GPS I noticed a 3 MPH difference between the speedo and the GPS.

Car speedo reflects 73 MPH and GPS reflect 70 MPH.
Car speed at 65 MPH, the GPS reflects 62.

Kind of curious if anyone else notice a difference?

I verified the proper tire spec's are accurate, and I know a GPS can be subject to speculation..
I also used the GPS on my iPhone and it reflects that same speed as the Garmin GPS..

T