I know this sounds weird, but the speedometer, trip odometer and the odometer are all separate instruments, geared off the one speedometer cable. If you've verified that the trip and regular odo's have the correct reading, then the instrument is correct for the car, and well matched to the tires and diff. That means that the only problem is that the speedometer is reading low. That can be easily adjusted by pulling the needle and repositioning it slightly. Before you begin, use a short length of speedometer cable to drive the speedometer with an electric drill. Not the speed reading. Then just adjust the needle so that it reads 5mph faster.