I should mention that speedometers of cars of that era always read high, although usually by less than 10 MPH at 60. But 5-8 was not unusual. Cars today still read at least a little high, even though with digital speedometers, they could be made virtually exact (as long as you're careful about tire pressures, anyway). The idea is that it's better to err on the high side--if you get a ticket for doing 72 in a 65 zone, because the speedometer reads 10% low, you're not going to have fond feelings toward the manufacturer. Also, with changes in tire type and size, over the years, plus drivers' carelessness in keeping the pressures right, making the speedo read a little high gives some room for all that.