silver79

I switch my TH350 to a 700R, the speedo is of. I have 255/60/15 tires with 3.55 gears. At 100 KPH the speedo reads 119 what gear should I have. I don't know what is in there now.

'75

http://www.2004r.com/speedoCalc/speedocalc.shtml

7T1vette

You need to know what [inner] speedo drive gear is fitted to the output shaft of the transmission. Knowing that, you can determine if you can correct your error with only a change to the speedo driven gear. Otherwise, you will have to change both gears.

The best time to select and install the speedo gears in an auto tranny is before you install it. You can use the speedo gears in the TH-400 unit to help determine the correct gears for the new tranny.

19 mph error at 100 mph (actual) is a 19% change. You will most likely need to change both the internal and external transmission speedo gears.

couperdecar

I have the same combo.
The gears come in different colors. Go to a tranny shop and get the dark green colored gear.

69 Chevy

Each incremental step for the colored plastic driven gears in my Muncie gave a 3 mph difference. I would expect a slushbox to be similar.