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#1 |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
Posts: 333
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Rear End Gears 1968 GMC - Need advice
Hi All,
I picked up a 68 GMC recently with a 292 - and 3 spd on the tree. It is geared real low 4:10. I was wondering what a better ratio would be with the 3 spd. Even going 55 Mph seems like a chore for this baby. I am in 1st gear for maybe 5 seconds lol. 3:08 ? Do they make something even higher? ![]() |
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#2 |
Registered User
Join Date: Nov 2014
Location: Lincoln City DE
Posts: 1,292
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Re: Rear End Gears 1968 GMC - Need advice
3:08 is high as you ever wanna go
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#3 |
Registered User
Join Date: Apr 2017
Location: Colorado Springs
Posts: 129
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Re: Rear End Gears 1968 GMC - Need advice
I would say 3:73 is highest you wanna go with a 3OTT and a 292. Any higher and that truck is gonna be an absolute dog getting up and going.
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#4 |
Registered User
Join Date: Dec 2011
Location: winnipeg
Posts: 1,107
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Re: Rear End Gears 1968 GMC - Need advice
Wow the ants in your driveway must hate that truck having to go all the way around vs under lol. A 3.07 gear would be good to get the rpm diwn
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#5 | |
Senior Member
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Location: Falls City, Nebraska "100 Miles From Nowhere"
Posts: 2,219
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Re: Rear End Gears 1968 GMC - Need advice
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Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" ![]() "Be American, Buy American" ![]() |
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#6 | |
Registered User
Join Date: Jan 2011
Location: Townsend MT
Posts: 1,725
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Re: Rear End Gears 1968 GMC - Need advice
Quote:
I guess a better question would be how fast do you want to cruise? |
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#7 |
Senior Member
![]() Join Date: Oct 2009
Location: Godley, TX
Posts: 17,964
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Re: Rear End Gears 1968 GMC - Need advice
Sounds like you are set on keeping the 3 OTT? If not, a kill 2 birds with 1 stone approach would be an o/d trans - improve low end torque multiplication for your 6 and reduce highway rpm. If committed to the 3 OTT, perhaps a period o/d unit is an option also. GV o/u an option, but a costly one. If only interested in a rear gear swap I’d settle on a 3.42 for a compromise. 3.73 will still probably still wind too high for you on the highway and a 3.08 will suck for low end acceleration.
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#8 |
Custom Sport W/T
Join Date: Nov 2010
Location: Tarpon Springs,Fl & Annapolis,Md. ....................Quaint drinking villages with a fishing problem
Posts: 898
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Re: Rear End Gears 1968 GMC - Need advice
Your truck won't pull the sheets off the bed with a 3.08 rear gear. Like Ironangel said you will be buying a clutch every payday.
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#9 |
Registered User
Join Date: Sep 2011
Location: Ark City, Kansas
Posts: 3,335
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Re: Rear End Gears 1968 GMC - Need advice
The factory had good reasons for their gearing choices.
I ran a 2.73 in my 67 Bel Air for years. Car had a built 283 and TH350 lockup. It perfromed OK, really needed a 3.08 (which it came with originally) OR a small block 400. Proper gearing for the engine size allows ease of operation and fairly good MPG. All dependent on usage. I would NOT touch what you've got. One thing I've learned, is to lean towards performance and drive-ability with a heavy vehicle. You can get the rev's down a bit, and a fuzz more MPG, but it won't be much and will NOT be worth the effort........... |
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#10 |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
Posts: 333
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Re: Rear End Gears 1968 GMC - Need advice
Thanks all for the pointers! I kinda wanna keep the 3 on the tree because you don't see it everyday. I don't wanna be buying a clutch every payday either. Getting up to 60-65 is really as fast as I wanna go. But it's so wound up and roaring going that fast I just wanna get the RPMS down a bit and not be a dog getting up and going. I don't plan to drop a V8 in this. My old man rebuilds engines and has always dreamed of hopping up a straight 6 so this will be his chance.
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#11 |
Registered User
Join Date: Aug 2016
Location: Lake Forest, CA
Posts: 2,177
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Re: Rear End Gears 1968 GMC - Need advice
I changed from 3:73 to 3:07 and it is awesome. Didn't lose any power or driveability.
Im thinking the guys who talk bad about the 3:07 have old, tired, 6-cylinders
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"Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC |
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#12 |
Senior Member
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Location: Conyers,Ga.
Posts: 7,287
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Re: Rear End Gears 1968 GMC - Need advice
You didn’t mention if truck has 12 bolt or Dana rear axle. Leaf or coil spring. Dana has few different options vs 12 bolt.
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#13 |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
Posts: 333
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Re: Rear End Gears 1968 GMC - Need advice
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#14 |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
Posts: 333
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Re: Rear End Gears 1968 GMC - Need advice
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#15 |
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Join Date: Aug 2016
Location: Lake Forest, CA
Posts: 2,177
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Re: Rear End Gears 1968 GMC - Need advice
Ah, then your next thread should be "swapping to a small block"...
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"Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC |
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#16 | ||
"I ain't nobody, dork."
Join Date: Sep 2007
Location: Whidbey Island, Washington
Posts: 8,940
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Re: Rear End Gears 1968 GMC - Need advice
Hi Matt!
Gary
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'cuz chicks dig scars... My 1972 GMC 1500 Super Custom (Creeping Death) "long term" build thread. The Rebuild of Creeping Death after the wreck Quote:
Quote:
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#17 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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Re: Rear End Gears 1968 GMC - Need advice
If you aren't sure it has a 12 bolt, are you really sure it has 4.10 gears?
My '67 had the 292 and 3 spd with 4.56 gears. It would do 60 on the freeway, in the slow lane, but the engine was screaming. 65 was doable, but not advised. Changing to the SM465 4sp was a slight improvement, but obviously not in high gear. The change to 3.54 gears was a night/day improvement. 75 on the freeway is now a breeze. I realize my 4600#, 3/4 is heavier than your 1/2, but I think 3.08 gears would be a mistake. Even though it would do 85, I would expect acceleration in the 55-75 range to be really flat. Loosing momentum on a hill or stopped at a light on a hill, would be a bear.
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'67 GMC 2500, 292, 4spd, AC |
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#18 |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
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Re: Rear End Gears 1968 GMC - Need advice
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#19 | |
Registered User
Join Date: Apr 2008
Location: Buckley, Washington
Posts: 333
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Re: Rear End Gears 1968 GMC - Need advice
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#20 |
Senior Member
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Location: Godley, TX
Posts: 17,964
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Re: Rear End Gears 1968 GMC - Need advice
I guessed at your tire diameter from the pic and your trans type for a 68 3-spd - here are some numbers for comparison (if I guessed wrong, let me know and I can re-run, but this should give you an idea. No matter what 3-speed you have, it's going to have a 1:1 3rd gear):
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