So the last thing I need to get right is my speedometer gear. To explain and keep it short my speedometer works but its like 10-20 miles off while driving. The cable is fine but I noticed it leaking at the part circled below I'm trying to fine new seals for both ends. What I want to know is if my speedometer is off should I just replace the part? Also what's the name of the part it looks like a speed reducer. I can't even find that part it seems my grandfather had a reducer installed but hell it still has my speedometer off. I'm thinking I should scrap the reducer and get the correct speedometer gear bullet. Thoughts?

 

What gear are you running and what tire?

 

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Randy's table is the way that the factory (and most owners) figure out what gear they need. You need to know the rear axle ratio and tire size as he mentioned.

There is another way that you may find easier.

Before doing that, you need to make a few tests.

Let's start with the assumption that, if your grandfather used the Stewart-Warner gear box, it is required. As I recall it can be installed either direction--to increase the speedo reading--or to decrease the speedo reading. Try it both ways and try it removed.

Now for the other way to do the job.

Stat with whichever condition makes the speedometer more accurate--Stewart-Warner as an increaser, Stewart-Warner as a decreaser, or no Stewart Warner.

You need to know the color of the speedometer gear that is currently in the transmission--the end of it drives the speedometer cable. You can see the end of it in one of the photos but it is so dirty I can't make out the color. Spray some brake clean on it and get the color--Randy's table lists the possible colors. You can also remove the gear and count the teeth.

Get a stop watch and drive on an Interstate with mile markers. Have a helper in the car. Your job is to keep the car at exactly 60 mph as shown on the speedometer. The helper's job is to time between mile markers. Time more than one pair of markers because the markers sometimes are not exactly a mile apart--such as when one would normally fall in the middle of an exit or entrance ramp.

Let's say the number of seconds you measure between mile markers is 73. And your speedo gear color is Green-42 teeth (assuming you have a Hydra-Matic transmission). Multiply 42 (teeth) by 60 and divide by 73 (seconds). You would get 35 (teeth for the new gear). You would need an Orange gear.

Let us know how your project turns out, whichever way you choose to go.

 

That adapter is stamped 1.125? Try running without the adapter and see what the difference is, then try an adapter of a different ratio. Also you could use a GPS to check your speed against the speedo.

 

Thanks guys. Headed to the shop now. Thing is I just got the engine rebuilt so I can't go over 55 for 500 miles. I will have him change the speedometer gear and scrap the reducer. The shop referred me to a transmission\speedometer specialist that knows about this. Lets see if the speedometer gear without the reducer helps if not I'll take it to the them. Thanks for the chart as well!! Printing now to bring to them.