Thread: Speedometer gears-TH350, which one do i need?

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I'm hoping maybe someone else around here has run the same combo and has played the speedometer gear game. I took the roadster out today for its first official shake down run... Aside from a short list of minor issues, it seemed to do pretty good... Has a slight shimmy at highway speeds, that i am HOPING is in the tire/wheel balancing... and the steering wheel is way off center when going down the road striaght, but other than that, it ran pretty nice.

Anyhow, one other problem is that my speedometer is way off... I have 285/60 15 rear tires, (radials ) just about 30" tall, and a TH350 trans... anyone happen to know what speedometer gear i need to get a relatively accurate speed reading? Right now it seems to be about 15-20mph on the slow side. I don't even care if its 100% accurate, would just like to get something closer than what i got, so my odometer is a little more accurate as well. Thanks in advance for any help or ideas. -JB

This is not a Turbo 350 ... but the principal is the same ...
You need to know the rear end gear ratio and the height of your rear tires ...

The speedometer gears are color coded ... so you will know the number of teeth on the shaft ... and then adjust. You may have to change both ...

then go here ...

http://www.bgsoflex.com/speedo1.html

Simple really ... to get it dead on ...
Look here ...

http://www.transmissioncenter.net/sp...____va.htm#350

Thanks, but how would I know how many "speedometer revolutions per mile" i need to type into that chart? And is the plastic color coded gear the drive dear or the driven gear? My car has a 30" tall tire and a 3.00:1 rear end ration btw.

tire rev per mile 20168/30 = 672
672X 3.00x drive teeth / 1001= driven teeth
drive is on the shaft driven on speedo cable what color is the drive gear?

At a minimum you will need know exactly how many MPH at a given speed (say 55 MPH) your speedometer is off and the number of teeth on the driven gear.

If the speed difference is too great to change with the driven gears available, you will have to change the drive gear on the output shaft of the trans (requires removing the tail housing).

Some Chevy trucks used to use an external speedometer adapter that can also be used to correct the speedometer difference (they are marked on the side with the ratio).

Joe
Sometimes the speedometer revolutions per mile is printed on the
bottom of the speedometer face in very small letters. If not, turn the
cable and count the turns till the odomrter goes one mile.
Dave

Originally Posted by joeybsyc

Thanks, but how would I know how many "speedometer revolutions per mile" i need to type into that chart? And is the plastic color coded gear the drive dear or the driven gear? My car has a 30" tall tire and a 3.00:1 rear end ration btw.

Type in 1000 ...

There are 4 different gears available for the driver ( the output shaft ) and they are either a 7, 8, 9, or 10 ...

With your rear in the ( 3.00 ) ... and your tall 30 inch tires ... you need the driver/driven ratio to be 2 to 1 ...

That means ... the number on the driven ... needs to be 2 times what is on the output shaft.

If your output shaft has a 7 on it ...
You need a 14

If your output shaft has a 8 on it
You need a 16

If your output shaft has 9 on it
you need a 18

If your output shaft has a 10 on it
You need a 20



There are no 14 tooth and 16 tooth available
The available driven gears are 17, 18, 19, 20, 21, and 22
So you need at least a 9 on the output shaft ...

I suggest a new bullet also ...