Differential

[speedbucket]

Hey guys, I have a 1967C 20 with an eaten full floater rear end. I love everything about the truck except for the mileage. Is there any way that I can switch the rear end to a higher ratio so I can get better gas mileage? Thanks in advance

[gmc684x4]

The best you can do is 4.10

There is some ultra rare 3.90 that was made years ago there pretty much unobtanium

Run a a taller tire and or a over drive

[Steeveedee]

Do you know what the gear ratio is? Is it a leaf or coil rear suspension?

[speedbucket]

Originally Posted by Steeveedee

Do you know what the gear ratio is? Is it a leaf or coil rear suspension?

It’s coil with leaf helper springs

[speedbucket]

It goes 50-55 then pretty high revs which for a 327 is somewhat acceptable. I think it’s 485 or some other camper or trailer haulyratio

[speedbucket]

65 mph is pretty high revs

[speedbucket]

Overdrives are $2300

[MJN]

As it was stated above the best you can do for that rear axle is 4.10's. Standard gear ratio for these was 4.56- thats most likely what you have. If you want to confirm there are numbers stamped on a flat area of the third member above the pinion or you can pull the cover and count the gear teeth.
Besides an overdrive unit or 5-speed transmission is to swap the rear axle out and that would either require fabrication to make a 14-bolt (they can be found with 3.73's) work with your coil springs or trying to find a Dana 60 from a coil spring truck that would bolt in and having a different gear ratio ring and pinion installed to get the ratio you want.

[57taskforce]

Originally Posted by MJN

As it was stated above the best you can do for that rear axle is 4.10's. Standard gear ratio for these was 4.56- thats most likely what you have. If you want to confirm there are numbers stamped on a flat area of the third member above the pinion or you can pull the cover and count the gear teeth.
Besides an overdrive unit or 5-speed transmission is to swap the rear axle out and that would either require fabrication to make a 14-bolt (they can be found with 3.73's) work with your coil springs or trying to find a Dana 60 from a coil spring truck that would bolt in and having a different gear ratio ring and pinion installed to get the ratio you want.

Though not the most common ratio, in the 80’s the 14 bolt full floater could be had in 3:42 gears. That would be a nice improvement if you run a lot of Highway miles. You will need to strip the housing of all its original leaf spring brackets and re weld the trailing arm stuff back on as well as a track bar mount, but totally doable.

[GASoline71]

What are high revs? 327's are built to take high revs from the factory as all SBC's are. And by high revs I mean somewhere in the neighborhood of above 5k RPM's.

However... running at 3,500 RPM's will suck a bit of gas.

Gary

[Mike C]

Dana 60 coil spring rears are out there and big block trucks had 3.55 gears. You can get aftermarket 3.31 gears as well. As noted above, brackets can be purchased and installed by a competent welder to put a full float or semi float 14 bolt in the truck with all the gear choices those allow.

[kwmech]

Originally Posted by 57taskforce

Though not the most common ratio, in the 80’s the 14 bolt full floater could be had in 3:42 gears. That would be a nice improvement if you run a lot of Highway miles. You will need to strip the housing of all its original leaf spring brackets and re weld the trailing arm stuff back on as well as a track bar mount, but totally doable.

Although rare, 3.21s could be had in the van 14 bolts. I snagged a set a few years back and put them in a truck 14 bolt that was getting a 3/4 ton conversion in a diesel truck that kept breaking 10 bolts.

[speedbucket]

Thank you all for the excellent info.
I will look into these options!
Greg