Speedo gear help

[2001convertable]

Hi, I changed the rear on my 1959 1/2 ton to a 338 from a 390 gear set. The truck has a 235 engine with a 4 speed manual. Does anyone know which speedo gear I would need to sync the speedo the actual speed? Thank you

[mr48chev]

Not off hand if you know the percentage it is off at a certain speed it is a bit easier to figure out.

Say you run down the road and your speedometer reads 66 mph and the speedometer app on your phone reads 60 you would be 10% off.
This lets you accurately calculate what gear you need.

I'm not finding the one I want but there is at least one chart that shows the speedometer gears for the older Chevy stick transmissions.

There is another way you can go and that is with a calibration box that connects to the speedometer housing on the trans and the cable connects to it. It has gears inside that correct the drive speed. They are real popular with 4X4 folks who swap tires around. This is a Toyota site but explains the use of one of the boxes. https://www.yotatech.com/forums/f116...l-pics-195986/ Most semi truck shops have them and I have found them on 4x4 trucks in the wrecking yards. You still have to put the right gears in them. https://www.yotatech.com/forums/f116...l-pics-195986/