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| | | | Joined: Sep 2010 Posts: 695 Shop Shark | | Shop Shark Joined: Sep 2010 Posts: 695 | Guys,
I apologize, this has probably been asked many times before.
I have an SM420 in my truck and I believe I need a Reducer for my speedometer cable.
The question is, what is the best way to determine what ratio I need?
Thanks in advance!
Jerry | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | Jerry, Will you be looking for an adapter, or do you hope to find a pair of original gears (drive/driven gears) that will work with your SM420? Good luck finding the appropriate drive/driven gears. Here are instructions for a ratio adapter. You can nowadays more easily find your ratio/correction factor by using a GPS to find your true MPH. A ratio adapter can then be made to result in a better speedometer reading.
Last edited by tclederman; 05/29/2015 1:57 PM. Reason: added link to ratio test - oops
| | | | | Joined: Sep 2010 Posts: 695 Shop Shark | | Shop Shark Joined: Sep 2010 Posts: 695 | Thanks guys.
Tim, I'm looking at an adapter, I just don't know what ratio I need for the speedometer. Did I miss your link to the instructions?
I don't have a working speedometer right now.
I bought a matching set speedometer gear and pinion and installed it in the SM420. When I took the truck on it's maiden run, I saw the speedometer peg out at top speed and I was only going about 30 mph. It twisted off the speedometer cable. I just figured I have a seriously wrong ratio.
Maybe the cable was faulty?
Jerry | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | Sorry about the link, Jerry - it is up there, now
I have a ratio adapter that has almost no error at 60-65 mph.
I did Method 1, but I used 5 x 10 measured miles and averaged the results. That adapter is on my Suburban: SM420 4-speed, 3.55 rear conversion, and 31.5" tires.
I will be doing one soon for the 54 we are restoring: Hydra-Matic transmission, 3.55 rear conversion, and 31.5" tires. I'll use a GPS unit for measurements/data for this adapter.
Your cable might have gone bad due to poor lubrication? | | | | | Joined: Sep 2010 Posts: 695 Shop Shark | | Shop Shark Joined: Sep 2010 Posts: 695 | Thank you Tim!
Since I am needing a new speedometer cable anyway, I'll buy one and try the 52'-9 1/2" measure method first.
If it checks out, great... I only need to connect my new cable to the speedometer and I'll determine that my old cable was the problem after all.
If it doesn't check out, I'll have the tools to determine correction. One handy tip noted was the "ten turns" for the cable at that distance.
Good luck with the '54!
Jerry | | |
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