When the speedometer reads 50, GPS has me @ 53mph. When Speedometer reads 60, GPS has me @ 68mph. Speedometer won't go above 65, even though GPS has me @ 85mph. The odometer is innacurate as well. GPS and milemarkers are at 150 miles, odometer reads only 120 miles. Any ideas?
Sounds like a bad signal from the speed sensor in the transmission. If you have a scan tool you could try monitoring the vehicle speed and see if it has a steady reading.