I have a 1970 Nova with a th-350 transmission. I had 234/60r14 tires and just switched to 275/60r15 tires. I also had 2.73 rear end ratio and just switched to 3.73. My speedo is way off. I looked at the speedo gears and the drive gear is a 7 tooth and the driven is a 20 tooth. I tried to do the formulas I found on the internet but they say to step down to a 19 tooth. That would make my speedo even more off. Right now I think my speedo says 85 when I am going 60. Can anyone tell me what speedo gears I need to make my speedo read correctly. Thanks a lot!

You can try www.tciauto.com/tech_info/speedo_gears.htm , or plug your tire diameter & axle ratio into a formula at www.novaresource.org/speedo.htm & www.novaresource.org/speedo2.htm & pick the appropriate gears from the charts.

Gears are cheap at the Chevy place.

Most awesome reply "jimfulco"
Very helpfull, just what I was searching for.

I'm trying to correct an 8mph error in a TH-400 trans, this info will help greatly!

Looks like it's easiest to ONLY change the DRIVEN gear and not worry about the drive gear. Just get it as close as possible that way.

Thanks