Heres how I fixed mine in my '37. The speedometer had set outside in the dash for 30 years so it was pretty crusty. I gave it a good cleaning with brake cleaner, then a couple of washes with WD-40 blowing it out carefully with an air hose till the WD-40 came out clean. I then oiled it lightly with sewing machine type oil. I have a hand held tachometer made for small engines, the type with a rubber cone on the end that reads off the end of a rotating shaft. I clamped a drill motor in the vise and set the rpm to 1000 rpm using the hand tachometer. GM used 1000 cable rpms to indicate 60 miles per hour. With the drill running at 1000 rpm my speedometer was reading slow, yet the odometer was clicking off 1 mile per minute just as it should. I pulled the needle off the gauge, I had it off while adding new decals, then set it back on the shaft but didn't push it down tight. I put it back on the drill running at 1000 rpm and carefully moved the needle to read 60 mph. I know it's not accurate at any speed but 60 mph, but its plenty close for my driving.
Knowing the speedometer and odometer were accurate at 60, it only took a 10 mile drive out and back to see how much I needed to adjust the driven and drive gears in the transmission. There are plenty of online calculators to use for the gear selections. Using highway markers, mine is still 2% slow with new tires, I can't get any closer due to the limited gear selection, mines is in between gears, either slightly to fast or slightly to slow. The adaptors can get right on the money. I have driven through plenty of the speed checker radar trailers here in the Kansas City area and have found speeds to match my speedometer most all the time, or as close as I can see when wizzing by them!
Last edited by Joe H; 09/15/2024 3:27 AM.
