I need help figuring out how to reduce my speedometer by 25%, odometer shows 2.5 tenths to fast in one mile. I pulled the driven gear and housing assembly out of my TH400 and the driven gear has 42 teeth (green) and I think the drive gear has 15. I tried the TCI website and using their calculations I need a 13 tooth driven gear. This doesn't make sence to me. My Chevelle has M/T 29 x 12.5 x 15 tires (diameter is 90"), 4.11 gears with a turbo 400. Anyone have a similiar setup that they were able to figure out?

As a rule of thumb:

You need to add teeth to your driven gear to slow down a speedometer/odometer. If you got an existing 42 tooth driven, the T400 has up to a 45 driven. Not enough to do what you need but, it will be closer and you won't have to remove the tail extension.

It is time to drive your car, the weather says so!

Regards, Jody

If its a 15 tooth drive gear in there , I used this http://www.bgsoflex.com/speedo1.html to figure it out . And you need a 43 tooth driven gear TCI p/n 43 purple # 880031 . Now thats if your drive is actually 15 tooth and grey oem # 8629547 . :)

To answer your question as to why your calibration didn't make sense:

Probably your input information was off on the TCI calculator, 90" is your circumference not your diameter. To know the diameter you need to divide pi (3.14) into circumference: 90 divided by 3.14 = 28.66" tire diameter

Try the formula again............

You cannot guess or think the drive gear is a 15 tooth count, it must be known for the correct driven gear. If you are unsure of the drive gear tooth count and you don't want to dissassemble the tail extension, adding teeth count to the driven will slow it down. You can go up to a 43, 44, or 45.

Regards, Jody

Thanks for the info guys!

I did have part of my info wrong. I was using circumference instead of diameter. After recalculating using a 15 tooth drive gear I came up with a 43 tooth driven gear. I know changing from a 42 to a 43 is not going to reduce it the 25% I need so I recalculated using a 17 tooth drive gear and using their calculator I got N/A so I used my calculator and got 48. Now I know they don't make a 48 tooth driven gear so am I stuck replacing the drive gear and if so how difficult is it to do with the tranny still under the car?