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| | | | Joined: Mar 2006 Posts: 32 'Bolter | | 'Bolter Joined: Mar 2006 Posts: 32 | I have a GM 12 bolt 8.875 on my 49 1/2 ton. Current gear ratio is 3.73. Looking to change to 3.90. Does anyone know where to get a truck gear set that is 3.90. I see a lot for cars, but none for trucks. | | | | | Joined: Apr 2020 Posts: 322 Professional procrastinator | | Professional procrastinator Joined: Apr 2020 Posts: 322 | I am not sure if what I have will work for you. I have a 3.90 ring and pinion set I found in my parts yesterday. If you PM me your email I can send you some pics.
1947.2 GMC 1/2T SWB panel
1947.2 GMC 1/2T long bed
1948 GMC 1/2T short bed
| | | | | Joined: Feb 2004 Posts: 29,755 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,755 | Is there a good reason for making such a tiny change in ratio? The same difference in engine RPM could be accomplished with a slightly lower profile tire, with a lot less hassle and expense.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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Love your enemies and drive 'em nuts!
| | | | | Joined: Sep 2011 Posts: 1,452 'Bolter | | 'Bolter Joined: Sep 2011 Posts: 1,452 | 3.90 was a ratio wasn't used past '62.
Integral housing axel such as your "12 bolt" don't have much in common with older units.
Last edited by BC59; 08/04/2025 11:27 PM.
BC
1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc
1949 GMC 250 project in waiting
1960 C60 pasture art
Retired GM dealer tech. 1980 - 2022
| | | | | Joined: May 2005 Posts: 8,983 Sir Searchalot | | Sir Searchalot Joined: May 2005 Posts: 8,983 | IMO 3:73 is the perfect ratio for acceleration, torque, gas mileage, speed, hiway...it's a compromise.
Last edited by bartamos; 08/05/2025 4:29 AM. Reason: condensed
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
| | | | | Joined: Mar 2010 Posts: 10,272 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,272 | Put me down for Hotrod's suggestion of slightly lower height tires. An inch lower tire will give you the RPM change you seem to be after.
There is a calculator available online which takes into account RPM, tire height and rear end gear ratio. You plug in what you currently have with your current tire height, then replace your current tire height with shorter tire heights until you end up with the RPM you are looking for.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Mar 2006 Posts: 32 'Bolter | | 'Bolter Joined: Mar 2006 Posts: 32 | Thanks. This answers a lot of my questions | | |
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