Yeah I had a problem similiar to that. The original speedometer in the truck would read up to about 25 mph or so, then it would just flip over to the other side. I finally found one in an old Chevy sedan delivery that I took the head out of and mounted behind my speedometr face.... not sure how accurate it was, since the truck speedo read to 90 mph and the sedan delivery read to 100 mph.....

John