Reading a couple under is common with lots of cars. Auto makers want to make sure you are not going faster than what it says, so most will make the speedo a little on the fast end. Most of my cars read 2-3 MPH faster then the GPS.
Question 1: what tire and size are you running?
Question 2: What did you use to confirm that you are running slower? GPS is more accurate then other methods (IE: police speed carts or driving behind someone else and getting their speed compared to yours).
Celica Intake, Tein H Techs on KYBs, Power antenna, Heated power mirrors, OBX Header and Underdrive crank pulley