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Discussion Starter · #1 ·
:beers: Hey TC'ers it has been awhile since I have posted according to the ddescpition at the top of my page, so here goes.

I have new Autometer gauges and would like to get the speedo working. However the old speedo was good, but did not work when installed. (I spun the cable with drill and held it at every increment of speed) that is how I determined that I had a good speedo but a problem with the drive gear and or the driven gear.

Please Solve this: What would be the proper gears to install in a th350? I want to replace both.

What is the easiest way to do this?

Explain the mathematics behind deciding the proper gears, ie How do I figure this out?

Do you need this info to solve?

Tire size, rearend gear, engine, type of speedo?

255/60-15, 2.55, 327/300hp, electronic speedo gauge.

Tia :cool:

189 Posts
A re-post from another thread....

SGR= speedometer gear ratio

SGR= 63360 x axle ratio / 3142.6 x tire diameter

To determine the number of teeth you need on the driven gear, multiply the SGR x number of drive teeth on your mainshaft

rear: 3.08
tire diameter: 26"

SGR = 63360 x 3.08 / 3141.6 x 26 = 2.389

For an 8 tooth drive gear:

8 x 2.389 = 19 teeth on the driven gear.

PS - the gearset used for the M20 and TH350 are the same. The plug, o-ring, and retainer on the transmission is the same also. Just determine what gear you have on the mainshaft and buy the appropriate driven gear. They usually cost about $5-10

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