Ok, any one with a '03 GT wagon or sedan notice that there spedometer was consistantly around 5 mph slower reading than what the GPS says you are going?

If you have larger than original tires on there, it can do that. My lifted Impreza has 28 inch tires and is off by about 5mph on highway.

Nope stock 205/55R16 Nokian WR with stock 16's to fit over the big brake of the GT.

usually, the complaint is the other way.

 

could your car have had some drivetrain components swapped in the past? Are you tires over inflated.

 

most people find modern soobs to indicate fast by 2.5% or so.

 

Subaru was even involved in class action suit and had to extend some warranties because the odometer indicated higher-than-actual mileage.

is it always this way? 

could the GPS not be accurate?

 

cars generally read higher than actual speeds, so if the GPS is accurate that is odd.

 

Legacy GT's have the highest final drive ratio ever offered in a Subaru, 4.44.  Base model legacy's have 4.11 final drives, so if the transmission was ever swapped that's a possibility, but that's highly unlikely.