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Username Post: SM420 spedometer gears        (Topic#354940)
Silver Supporting Member
Posts: 387
Loc: West Richland, WA
Reg: 02-05-17
01-23-19 08:35 AM - Post#2758004    

Does anybody have a source of speedometer gears for a SM420 transmission? My brother had his rear end rebuilt with a different gear ratio and now his speedometer is off by 20%. We just removed the transmission for a rebuild, so I thought I'd look for replacement gears to get it set up right. I know that these are rare, and that you can get a gear reduction to fix the problem, but I was hoping to find somebody that had a set to fix it with replacement gears instead. I need to open the transmission up to find out what's in it and then calculate what gear ratio we need, but I thought I'd ask the forum first if anybody had a source.


Valued Contributor
Posts: 3514

Age: 75
Loc: Sidney, B.C.
Reg: 12-03-05
01-25-19 09:56 PM - Post#2758226    
    In response to jktucker92

what ratio did you have in the rear and what do you have now. perhaps someone can come up with the right speedo gear

Silver Supporting Member
Posts: 387
Loc: West Richland, WA
Reg: 02-05-17
02-02-19 04:22 PM - Post#2758846    
    In response to sidworks

The truck originally had a 4.11 gear ratio, and the speedo gears have 20 teeth on the driven gear and 7 teeth on the drive gear. New gear ratio is 3.42, so it looks like I'd need one of the following sets if available. Not sure what the correct numbers would be, or even if they made speedo gears for a 3.42 ratio. Attached is a photo of the existing speedo gears.

driven drive ratio
21 9 0.428
19 8 0.421
17 7 0.411

Attachment: IMG_20190202_135005.jpg (3.19 MB) 32 View(s)


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