2001 Acura Integra Speedometer displays 5-9 mph faster than

Hello, I just bought a garmin GPS that displays current mph and when I compare the mph from my car's speedometer to that of the GPS, my speedometer is overstates it by 5-9 miles. I've noticed that the discrepancy increases the faster I go, when I am going below 40 mph the difference is relatively small 1-3 mph, but once I get to around 50 it gets into the 5ish range and when my speedometer is reading 90 mph, the GPS is displaying 81 mph.

I tested the GPS in two other cars to make sure that it wasn't faulty and both times it was right on, so I know that the problem is with my car's speedometer.

Thanks