[android 211]

My GTO's speedometer is way off. Reads very high. The car has the original 3.23 rear axle which was standard for automatic air conditioned cars. I've reviewed the service manual but can't quite figure out which gear I need to have in my Turbo 400 to make the speedo read right. The tires are approximately the stock size (225/14 70s) so which speedo gear would be right? How many teeth?

[Verdoro 68]

Someone probably knows off the top of their head, but in the mean time TCI has a calculator for speedo gears.

https://www.tciauto.com/speedometer-gear-calculator

I need to find the right one for my car too. I've got the wrong drive gear in the trans right now which complicates things.

[Verdoro 68]

This question inspired me to get my car finally dialed in.

Using the calculator above and presuming you have a 15 tooth drive gear, you'd need the red gear with 37 teeth.

For my weird case...by the powers of deduction I figured out that I have an 18 tooth drive gear ('77 Cadillac TH400) so there aren't any driven gears that will give me the correct reading with my 3.55s. To top it off, the drive gear is pressed on and pulling it is apparently a chore. After a little searching, I ended up getting a 17.6% drive reducer and using the 43 tooth gear. It's spot on now!

[dataway]

Had no idea they even made such a thing as that.

[android 211]

My car has a PT code Turbo 400. It came out of a mid seventies Grand Prix with a 400 2bbl. That car must have had a really tall rear end as even with a 38 tooth gear the speedo is still way off. I'm going to have to get an external adapter. Cheaper than pulling trans and changing the drive gear.

[Ben M.]

You don't have to pull the trans to change the gear, just the driveshaft and tail housing.



BUT you need to know what's in there first. My car had a 36 turn driven gear with a 21 turn drive gear. To get my 3.23 geared car to read correctly I changed the 21 turn drive gear to a 15 turn gear and it read spot on. If you know what your driven gear is you can do some math to determine what gear to install.

Or install the reducer box, that's an option as well (albeit more costly).

[Verdoro 68]

The reducer boxes are costly for what they are, but it’s a 5 minute install. Apparently there’s a special Kent Moore puller for the drive gear, but I couldn’t find one. Some TH400s use clip in drive gears so it’s worth taking a look to see what you have before you drop some money.

https://transmissioncenter.net/shop/...ratio-adapter/

[Held for Ransom]

Originally Posted by android 211

My car has a PT code Turbo 400. It came out of a mid seventies Grand Prix with a 400 2bbl. That car must have had a really tall rear end as even with a 38 tooth gear the speedo is still way off. I'm going to have to get an external adapter. Cheaper than pulling trans and changing the drive gear.

It probably has the 18 tooth drive gear ('74 PT) and needs to change to the 16 tooth drive gear to start out with. It should be plastic, making it easy to swap out. Then I'd start out with the Black 40 tooth driven gear.