[ml3126]

I have a 1970 GTO with a th400 trans. My speedometer is way off. Anyone know what color speedo gear I need with a 3.55 rear?

[b-man]

You need to count the teeth on the drive gear that's on the output shaft first off.

Probably either 16 or 18 teeth. Take out the driven gear housing and look inside, you will see the drive gear. Wipe it off with some solvent and mark one of the teeth with a Sharpie permanent marker. Turn the driveshaft slowly with the trans in neutral (car safely blocked from rolling of course) and count the teeth on it.

After you confirm the drive gear tooth count, figure out the rear tire height.

After that, you can calculate which driven gear you need to make your speedo accurate.

Sometimes you may need to change both the drive and driven gears to get the most accurate speedo reading.

TCI Speedo Gear Calculator:http://www.tciauto.com/tc/speedometer-gear-calculator

['ol Pinion head]

my GT-37's PQ coded turbo 400 has its original gray driven gear, original 3.55 12 bolt G70-14 tires. your '70 should use the same. Not sure where the driven gear changed. if the T400 is out of a much higher ratio vehicle, prob going to need to look that up.