t400 speedometer drive gear

[mcsteve]

i'm changing from a 2.56 to 3.31 rearend gear ratio. 1970 mc ss 12 bolt. my t400 trans is being rebuilt n my trans builder is not sure what drive/driven gears to use for the speedometer in the trans -  gonna use 26.5" tall rear tire (245 60 15).  i looked online at charts but are confusing  to use? any help would greatly appreciated! thanks, Steve

[riverdogs00]

lol....I went through this 8 yrs ago and kept my old gears. I thought, I'll just walk out and see what I have and its likely the one missing.  Nope ha 

I did the very same swap, although my tires are only 26".   I see that my stash has a 35 (I think that was the stock gear), then I have a 38,39, 40, 43 and 44 tooth gears

I did go to TCI's web site, and its pretty easy.  Pull the old speedo gear and adapter out of your existing trans, and count the teeth that are on the output shaft (inside the trans). If you put it in neutral, you should be able to spin the shaft. or, look at the color of the gear. There is a chart near the bottom of the link below that shows the color vs # of drive teeth for the turbo 400

then go to the calculator at the link below, scroll to the top of the page and key  THAT number into the first box  (# of drive teeth)

https://www.tciauto.com/speedometer-gear-calculator#400

Without knowing WHAT you are starting with ( number of drive teeth), its a shot in the dark.....which is what I did and now I have multiple gears left over ha.

 

 

hope this helps. 

 

 

 

[Scott S.]

I've used the same chart..... Worked good for my 200r4 as well.

[mcsteve]

thanks for the response. my old drive gear  ( in trans) had  18  teeth w 2.56 gears. my trans rebuilder said he gonna put in a 15 tooth n we'll figure the other out when car is done. Used tci formula n comes out to 37.75 teeth so maybe a37 or 38 will work. just needed the correct website n your explanation. Thanks again, Steve