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Jul 21st, 2025 | | | | Joined: Feb 2020 Posts: 20 Grease Junkie | | Grease Junkie Joined: Feb 2020 Posts: 20 | 49 3600
I was lucky enough to find a 4.10 differential to replace my original 4.57 gear set. So, I've been having to rely on a GPS to avoid speeding tickets. I'd like to swap the original #3689178 speedometer gears in my SM420 with #3689223 gears from a 1/2 ton. That should get me close enough on the final drive ratio to ditch the GPS.
My question is (and yes, probably a stupid one) can I just pop out the original 13 tooth driven gear an pop in the 12 tooth gear or do I need to swap the drive gear too. I ask because the drive gears in both sets have 4 teeth.
Thanks in advance,
Steve
1949 3600 w/ 57 235
| | | | | Joined: Mar 2019 Posts: 1,003 'Bolter | | 'Bolter Joined: Mar 2019 Posts: 1,003 | I don't really think, either way, weather your speedometer is 5 mph off or so, at 60 mph that your going to overwhelm the local "Barney Fife" wannabe. There are alot of other idiots out there that attract way more attention than a 70+ year old truck scooting through town at a high rate of speed, with total disregard to the posted speed limits. Well, unless we are in Mayberry... Lol... Don't worry. Someone will have the appropriate answer to your dilemma. But if they don't, you'll probably still be under the radar.😠| | | | | Joined: Sep 2020 Posts: 364 'Bolter | | 'Bolter Joined: Sep 2020 Posts: 364 | Jim Carter offers a speedometer ratio adapter, it is the $125. ᠁᠁external solution, follow 50mph ordering instructions, then connect᠁..
Just dont change your rear tire diameter᠁᠁᠁
Hah, mike beat me by a minute᠁
Last edited by Hanks custodian; 04/11/2023 2:15 AM.
Hank: 46 Chev 1/2ton shortbed
2023 Miata RF Club
| | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Or one can just check the speed difference with highway mileposts and clock, remember percent difference and figure out true mileage while cruising along?
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Jan 2002 Posts: 2,696 'Bolter | | 'Bolter Joined: Jan 2002 Posts: 2,696 | I use a ratio adapter which I got from a local speedometer shop a few years ago. Still running perfectly.
Craig My '50 Chevy 3100 5 window, '62-235cu, 3:55 rear My truck ....... Respect The Rust If I'm not working on my truck, '65 m00stang or VW camper, I'm fishing with the wife or smoking Salmon.
| | | | | Joined: Feb 2020 Posts: 20 Grease Junkie | | Grease Junkie Joined: Feb 2020 Posts: 20 | I'll reply to my original post here. I asked if I could just plug in a 12 tooth driven gear to replace the original 13 tooth gear without also replacing the drive gear. I tried it and the answer is "NO". If you need to change the speedometer gears to match a change in differential ratio or tire size you must change the speedometer gears as a set. so, anybody out there know where I can get a GM 3689223 gear set?
1949 3600 w/ 57 235
| | | | | Joined: Jan 2022 Posts: 1,986 'Bolter | | 'Bolter Joined: Jan 2022 Posts: 1,986 | Might be easier and cheaper to find a speedometer adapter with a 12 to 13 gearset.
Last edited by Bill Hanlon; 04/17/2023 11:56 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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