This won't explain your suddenly working tach but in the early '70's while living in the Catskills I bought a 1949 Chevy farm pick-up. The speedo didn't work but that didn't matter with my $50 truck purchase and a top speed of 45 mph. Miracle of miracles it began to work a year later when we moved to Buffalo. Had to make the 300 mile trip on all country roads - took two days--made me feel like pioneer in a Conestoga Wagon! There are some things you just shouldn't question. :-) -- Dave