My car seems to defy common wisdom. the speedometer was accurate when I bought it. The car had 4:10 gears , they were making noise, so I took it to Randy's ring and pinion. They replaced them with 3:31's (they didn't have the 3:27's). I ordered a new pink 19 tooth driven speedometer gear for it, assuming the gear had been replaced with a 23 tooth. When I went to put in the 19 tooth I was surprised to see that the one in the car was still the original 19 tooth gear. There doesn't appear to be a speedcal installed. I lifted out the shift bezel to see if I could see same wires coming through (nothing there). I can't see a little black box. The speedometer seems to be about 20% slow. Any thoughts on how this speed was calibrated for the 4:10's/ how I calibrate it back?