When Lacking a GPS I would drive the car on the Interstate from mile marker to mile marker. Needs to be over 25 miles though, the more miles the better the accuracy.

I divide the miles traveled on the odometer and the actual miles travel by the mile marker signs to get a percentage. If it is 20 percent too low I get a speedometer gear with 20 percent less teeth to speed it up.

That being said, John is right about speedometer heads being off. My method works for calibrating the Odometer with the hopes the speedometer will be right afterwards.