My speedometer lies to me
 

My speedometer always displays a speed that is somewhere between 6-8mph faster then my iPhone's Speedometer app shows me going. I have a 1998 Z28 with A4 that had 2.73 gears originally and the speedometer displayed faster then my phone's GPS app when I bought it. After I swapped the gear to a 3.73 it was Really off but I had it corrected with HP tuners. The odometer is now accurate again, as verified by my phone's GPS, but regardless if the speedometer is showing 10mph or 100mph it is always 6-8mph faster then the GPS. Anyone else's speedometer off like this? It confuses me since the odometer is accurate. It's like the marks behind the speedometer needle need to be turned clockwise 6-8 mph if that makes sense.