I recently rebuilt my speedometer for my '53 3100. I put a mechanism from an 80 mph unit in my 90 mph unit because the shaft on my 90 mph unit was broken where the needle attaches. These units are the same whether it's an 80 mph vs. 90 mph...right? I assumed the faces would be the only difference. The numbers on the speedo mechanisms had all the same numbers cast into the housings. The reason I'm asking is because my speedo operates smoothly but it doesn't begin to move until I hit about 10 mph. I have it set at zero. I begins registering when I accelerate to about 10 mph and then routinely reads between 10-15 mph less than my actual speed. Anyone know how to fix this or what's going on? The unit seemed to spin freely at the bench.

Thanks

1953 3100 | 1958 3/4-ton NAPCO