i replace my th 400 in my 1989 gmc r3500 with one i rebuilt and now my speedometer is off.should i take the speedometer gears off the old one and install them on the new one?

Yep.

While you're at it, count teeth on each of the speedometer to confirm.

Each tooth on the driven gear is about 3 mph

What? Explain please.

Example: If the speedo reads 9 mph fast increasing the speedo driven gear will decrease the speedometer reading 9 mph by slowing the rotation

Sorry I forget to mention to increase the speedometer driven gear by 3 teeth will slow the output to the speedometer and reduce the speedometer reading about 9 mph. I hope this helps. So use the speedo driven gear from the original trans. But like previously mentioned count the teeth on the driven gear.

OOOO KKKKK