Your original 1965 speedometer will read slow and the mileage should be accurate. Inside the speedometer is a magnet that becomes week over time allowing a slow reading on the needle. A speedometer repair place over charges the still magnet. Then with test equipment that has been calibrated to 60 cycles in the U.S. or 50 cycles Europe is then tun to 1000 RPM. The speedometer will read faster than 60 MPH. With a magnet charger the spinning magnet is zapped until the needle reads 60 MPH. The bushing in the head of the speedometer becomes worn over time, causing all kinds of issues.