Rear end gear swap help!!

[Awann99]

I’m running a stock 3:07 open rear in my truck with my th350. Since I am swapping to a 700r4 those gears will not work well with that transmission for a number of reasons. I have a spare 3:73 72 12 bolt and a unknown posi 77 12 bolt (both 5 lug). Would I be able to just have a shop swap the gears or is the 3:07 a different carrier and I would need to swap the whole rear end to get the gears I want. Thanks guys.

[firedemon]

you should be able to swap the carrier from the 12 bolt into yours as long as yours is a 12 bolt too . the different size series 2 to 4 I think it is all fit the same housing

[HO455]

Short answer is no your 3.07 carrier (commonly referred to as a 3 series carrier)will only go up to a 3.42 with a standard factory ring gear.
The medium answer is the 3.73 carrier will, go in to your original housing. You will have to do a positive identification on the 77 rear end to be sure you can use the posi carrier from it with the 3.73 gear set.
The long answer is below.

https://www.crawlpedia.com/thick_gears.htm

I have a 3.73 thick gear on a 3 series carrier in my Burban.

[Awann99]

So if I swap the carrier as well would it be possible. I’m thinking of installing a posi unit so I would need to change the carrier anyway. Thanks.

[gmc684x4]

Is it a posi or one of those gov bomb lockers ?

Never seen a gov locker that hasnt blown up

[RustyBucket]

[quote=Awann99;8763379]I’m running a stock 3:07 open rear in my truck Since I am swapping to a 700r4 those gears will not work well with that transmission for a number of reasons. Thanks guys.[/


I'd try it with the 700r4 before you change the rear gear. My'71 GMC stepside has a mild 350/700r4/3.07 rear combo and it drives fine. A 700r4 has a low 3.06 first gear and my truck isn't lazy off the line at all. It'll burn the right rear 275/60/15 T/A if you slap it too hard from a standing start. Mine goes into O.D. at about 50mph. Sometimes I bump the shift lever down one so it doesn't go into O.D. ... third gear on the 700r4 is a 1 to 1 ratio same as your current 350 turbo in top gear. Depends on what you do with your truck. I've had mine about 7yrs and it's a fun driver. EZ cruise on the Interstate and keeps up w/traffic good in town.

[Awann99]

I'd try it with the 700r4 before you change the rear gear. My'71 GMC stepside has a mild 350/700r4/3.07 rear combo and it drives fine. A 700r4 has a low 3.06 first gear and my truck isn't lazy off the line at all. It'll burn the right rear 275/60/15 T/A if you slap it too hard from a standing start. Mine goes into O.D. at about 50mph. Sometimes I bump the shift lever down one so it doesn't go into O.D. ... third gear on the 700r4 is a 1 to 1 ratio same as your current 350 turbo in top gear. Depends on what you do with your truck. I've had mine about 7yrs and it's a fun driver. EZ cruise on the Interstate and keeps up w/traffic good in town.[/QUOTE]

What HP does your engine make. I’m using a good wrench 350 that makes 195-260 HP. I will be using the truck as a comuter so mostly 65 mpg driving. What kind of mpg do you get? I was more worried about the pulling power and around town driving with the 3:07 rear.

[Boog]

My old 88 GMC had a 305/700r4 with a 2.73 rear end. That low HP combo ran ok overall. With that low HP I would have preferred a 3.42 but never bothered to change it. I learned to run it in 3rd gear around town and OD on the highway.

[RustyBucket]

[/QUOTE]

What HP does your engine make. I’m using a good wrench 350 that makes 195-260 HP. I will be using the truck as a comuter so mostly 65 mpg driving. What kind of mpg do you get? I was more worried about the pulling power and around town driving with the 3:07 rear.[/QUOTE]

I'd say somewhere between 195-260hp ... I'm assuming it's a prev. owner rebuild, I didn't do it. Original engine, cam change, H.E.I. distributor, 600 Holley on a Performer manifold, iron ex. manifolds... no hot rod motor. Seems like it uses a lot of gas, haven't checked mileage. Probably 10-12mpg. Truck has ps, pb, after mkt. A/C. In reality I'm not sure if the O.D. transmission is saving anything. First gear is a little stronger 'cause of the low trans. ratio.

[dennislbrooks]

I am not sure what tire you are running but if 29-31's using lockup in od will have a low (too low) rpm. I run 29's on my 42 and I ended up changing the rear gear from 3.08 to 3.73. The engine is about a 220hp 350 with a 700R4 and lockup.

I would move any perches and mounts and use the whole 77 rear end. All brake parts including the backing plates should interchange.

As far as correct carriers, I thot the breaking point was up to 3.73. 3.92 up takes a diff carrier. But HO455 may be correct.

[tim_mc]

I'm with Rusty & Awann and would suggest trying the 3.07 gears before swapping. I also run 3.07s with a 700R4 with hydraulic lockup and like it, especially on the highway. Sometimes for low speed local driving less than 45-50MPH, I just put it in D, which keeps it in the sweet spot for torque without dropping the RPMs too low. I have a fresh GM SBC 350 short block with mild Comp roller cam, Brodix IK-180 aluminum heads, Edelbrock EPS intake and AVS2 carb, full-length headers...maybe 280-300HP?

With that said, I plan on building a posi rear end, and *might* change the 3.07s to 3.42s (not 3.73s) as I'd still like to cruise the interstate without pulling too many revs.

3.07 w/26.5" tires ~ 1900 RPM @ 70MPH
3.42 w/26.5" tires ~ 2113 RPM @ 70MPH
3.73 w/26.5" tires ~ 2304 RPM @ 70MPH