I have a 1950 Chevy 3600. It has all original drive train, 216 and 4 speed tranns. My question is regarding the speedometer. I could tell it was reading slow, so I checked it against GPS. It seems that it is running about 12-15 mph slow. I'm wondering how this can be. I would get if it was running fast because someone maybe changed the gears in the differential, but this thing is geared really slow and I doubt that it could be geared any slower. As far as I can tell the differential is all original. The shop manual says it is 4.56 gears and I believe it. I'm afraid to take it over about 50mph (37 or so by the speedometer) as the engine is running pretty fast at that speed.

Any ideas?