[OrrieG]

Working from memory but when I put the TH350 & 3.74 rearend in my 59 the speedo was off. I ran it for 5 miles at 60 mph speedo reading on the freeway and timed how long it took. should have taken 5 minutes or 300 seconds. I divided the time it took in seconds by 300 and came up with the percentage over or under. I then pulled the plastic gear out of the tranny and took it down to the local Chevy dealership. He had a chart that listed the different colors and difference it changed the output. I got the one that changed it close to what I had calculated installed it and rechecked. Speedo was within 5 seconds for 5 miles so I was happy. This was in 85 so who knows if the current counter guys can help and if GM still makes the different gears. NAPA might have access to the same information too.

When I changed tires to a different diameter it changed again, I just did the test run and just mentally adjusted. I do the same for my Chevelle, I run short tires on mags in the summer, stock tall tires in the winter. Difference is 4 mph at 65.