I dont have GPS but I had assumed it was high tech dead on accurate, ain't so ???. What John said I would get the odo correct with the correct pinion then after that if the speedo ain't correct then it would need to come out & go to a speedometer specialty repair shop. I use 5 mile markers in a row on the interstate to check the odo and those portable lit up digital roadside MPH signs the city sets out on occaision to check the speedo