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Jul 21st, 2025 | | | | Joined: Apr 2010 Posts: 436 'Bolter | | 'Bolter Joined: Apr 2010 Posts: 436 | OK, I have now finished with the gear swap in my '62 C10. Speedometer is way off. How do I calculate what new gear I need to correct it?
I have the SM420 4Speed, 3.38:1 rear end, and P275/60R15 tires (28" diameter).
When I had the 3.90:1 gears, my factory speedometer was fairly accurate. Now it reads very low.
I've looked at a lot of online calculators, but they all ask what the number of drive teeth are. I don't know what they are for the SM420. Was there a standard drive gear in the SM420? | | | | | Joined: Oct 2017 Posts: 66 'Bolter | | 'Bolter Joined: Oct 2017 Posts: 66 | Google ratio adjuster. It's a little gear box that bolts to the Speedo cable mount on the transmission. There are a few places that sell them. It's far easier that finding the correct 420 gears.
Last edited by Truckfarm72; 03/23/2018 8:23 PM. Reason: Spelling
| | | | | Joined: Jan 2013 Posts: 1,518 'Bolter | | 'Bolter Joined: Jan 2013 Posts: 1,518 | Pull the output yoke off the back of the main shaft, the drive gear is pressed on that. Course you will have to remove the ujoint to get to it. Might be better to go to your local speedometer shop and have them install an adaptor like truckfarm said, drive and driven gears are getting hard to find for the sm trans plus it would be cheaper in the long run Dave.
Last edited by sstock; 03/23/2018 8:25 PM.
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
| | | | | Joined: Oct 2017 Posts: 66 'Bolter | | 'Bolter Joined: Oct 2017 Posts: 66 | I just noticed your in Sacramento. Commercial Speedometer is local to you. He will set you up. | | | | | Joined: Apr 2010 Posts: 436 'Bolter | | 'Bolter Joined: Apr 2010 Posts: 436 | Thanks for the information. I'll look into Commercial Speedometer. I was hoping it would be an east fix to just change the driven gear, but after more research on Stovebolt, it looks like I would need to change both drive and driven gear.
The old Powerglide and later the TH400 I had in my old '63 C10 was just change the driven gear to correct the speedometer. I don't really want to tear out the drive line again, so I'll see what Commercial Speedometer can do for me. At least research it online for now, figure out what kind of price range I'm looking at. I can work it into the budget then. | | | | | Joined: Oct 2017 Posts: 66 'Bolter | | 'Bolter Joined: Oct 2017 Posts: 66 | 50-100 bucks. You could junk yard dog it. GM put millions of those adjusters on rigs in the 70's-80's . A 1.12 or 1.15 would work. I have a whole box of them I have scrounged up. I still out of habit grab them at my pull and save yards.
Last edited by Tiny; 03/24/2018 11:40 AM.
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