Rear End Gears 1968 GMC - Need advice

[MattyBs71]

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?

[harpo231]

3:08 is high as you ever wanna go

[Keystone_Hate]

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.

[Ironangel]

Originally Posted by Keystone_Hate

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.

This! A 3:73 is an all around good gear in the case you went to a V8 or soup that six up...A 3:08 will have you replacing burnt clutches...

[1972RedNeck]

Originally Posted by Keystone_Hate

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.

3.73's isn't going to be a whole lot different than his 4.10's - I was thinking more along the lines of 3.54/3.42's - but then again, I am used to running 80 - 90 MPH on the highway...

I guess a better question would be how fast do you want to cruise?

[trac209]

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

[jocko]

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.

[CaptRMW]

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.

[68Gold/white]

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...........

[MattyBs71]

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.

[demian5]

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

[MattyBs71]

Originally Posted by demian5

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

That's what I have!

[demian5]

Originally Posted by MattyBs71

That's what I have!

Ah, then your next thread should be "swapping to a small block"...

[uncle]

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.

[MattyBs71]

Originally Posted by uncle

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.

Pretty sure it's a 12 bolt. It was leaf spring but has a 2 link on air bags now.

[GASoline71]

Hi Matt!

Gary

[MattyBs71]

Originally Posted by GASoline71

Hi Matt!

Gary

Hey Gary!!!!

[RichardJ]

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.

[MattyBs71]

Originally Posted by RichardJ

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.

I am not 100% sure on gears. That is what previous owner told me. I have only had the truck a week so far and haven't checked it yet personally.

[jocko]

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):