3.08 limited to 3.73 posi?

[rhc10]

Thinking of switching my 3.08 limited to 3.73 posi. Truck is running a very mild SB 350 with 700r4. Is it work the money to do this, am I going to notice a big difference? How will this affect highway cruising spped and rpms?

[mud.man.rj]

Running the same small block/700R4 with 3.73's in a 2wd I have, not sure of Rpm at 60mph, was fairly low, switched up from 3.08's also about 3 years ago, sure made a performance difference around town and still just idling on the highway, haven;t driven it for a few years due to the big 4x4 builds I have be busy with but will get back on the 2wd this summer again, perfect ratio with the 700r4 though.

[Fitz]

Originally Posted by rhc10

Thinking of switching my 3.08 limited to 3.73 posi. Truck is running a very mild SB 350 with 700r4. Is it work the money to do this, am I going to notice a big difference? How will this affect highway cruising spped and rpms?

The 8.875T carriers come in two sizes. Small for rear ratios from 2.73 tto 3.42 and large for 3.73 through 4.56. Since you are running a 3.08 to get to a 3.73 you will have to change out the posi carrier as well which means you get to add another $450.00 to the $185.00 you will spend for the new ring and pinion.

[GASoline71]

By 3.08 limited... do you mean limited slip? If so... that's what GM Posi-traction is... a clutch type limited slip.

Gary

[jocko]

Tire size will affect cruising rpm as well, but to give you an idea - here are the differences in what you'll see between a 3.73 and 3.08. 3.73 with a 700R4 is a very common combo that GM was fond of for a reason. Great bottom end perf with acceptable highway cruising rpms. 3.08 with a 700R4 will still work in 1st gear (i.e. still more torque multiplication with a 3.08/700R4 (9.42 final drive ratio in 1st gear) than a 3.73/TH350 (7.76 final drive ratio in 1st gear) - but highway rpm probably getting on the low side in the 55-60 mph range with a 3.08/700R4 combo, bordering on lugging the engine (i.e. it's only 1624 rpm at 65 mph...). Bottom line, you should see about a 350rpm increase at highway cruising speeds if you swap a 3.73 in place of your 3.08s. Anyway, here you go (I say go for it if money isn't a big issue (ha, but it always is, and Fritz is correct, not a cheap swap) - you'll find the 3.73/700R4 more streetable and won't lug the motor at lower highway speeds):

[rhc10]

Originally Posted by GASoline71

By 3.08 limited... do you mean limited slip? If so... that's what GM Posi-traction is... a clutch type limited slip.

Gary

Yes limited slip, but I was under the impression that what I needed based on what I want is a posi traction rear. Basically in lamens terms right now if I stomp on the gas only one tire spins which is why I thought I needed a posi unit. I'm kind of new at this any help would be appreciated.
I still want to go with the 373 so I guess I need to change the unit regardless.
Thanks

[BLT gmc]

Limited slip and Posi-traction are two terms that essential mean the same. It sounds like you have a one wheel peel. You may have an open 3.08 (non Posi) or you may have a 3.08 Posi that is completely out of tune (the plates that lock the diff are wore out or are not properly shimmed). To truly know you will have to remove the diff cover and look. Regardless I agree with Fitrz, I you are going to 3.73 posi you will have to change the carrier along with the ring and pinion. So it really does not matter much other than good trading material if it is a posi

[djgebert]

"am I going to notice a big difference? How will this affect highway cruising spped and rpms? "
I think you will notice a fairly big difference in city driving. It will be much quicker "light to light". I have my truck pulled apart right now, but I was running a 700r4 with 3.73 rear and a 383 and it was running under 2500 rpm at 70 mph on the highway and I could ignite the tires at will in slower city driving conditions.

[cdowns]

yes you will notice the difference// the best thing for you to do is take it to a shop drop it off and pay the $1200 to have your rearend built// in and out in a day

[mud.man.rj]

I agree about the take it to a shop if you can't do it yourself, I had a 10 bolt with 3.08's and 6 lug so swapped in a 12 bolt diff with a 5 stud pattern with a posi and 3.73's in it, but not easy to find a complete "good" diff. 3.73's with a 700r4 is a nice setup though.

[GASoline71]

Originally Posted by rhc10

Yes limited slip, but I was under the impression that what I needed based on what I want is a posi traction rear. Basically in lamens terms right now if I stomp on the gas only one tire spins which is why I thought I needed a posi unit. I'm kind of new at this any help would be appreciated.
I still want to go with the 373 so I guess I need to change the unit regardless.
Thanks

You probably have an "open" diff. You can pull the rear cover off your diff and if all you see are 4 side gears and a cross shaft inside the carrier then you have an open diff. If you see a bunch of clutch discs and springs in the carrier along with side gears, you have a Posi-traction (limited slip) rear end.

Posi-traction was just Chevrolet/GM's fancy way to say limited slip.

AMC = Twin-grip
Buick = Positive Traction
Ford = Traction-lock
Olds = Anti-spin
Chrysler = Sure-grip
Pontiac = Safe-T-Trac
Cadillac = Controlled Traction

All of the above mean the same thing... "Limited Slip Differential".

Gary