Rear end upgrade

[msco123]

I am thinking about putting a 3.42 differential in one of my supremes. I just wanted some feedback about this. Good or bad idea?

[77442]

I've got a 3.42 in my 1970 442. It takes off quite well with the 455 and TH400.:D Depending on what you want for gas mileage, that might be a factor. I was thinking about a 3.23 for my 77 442 with the 350. What are you going for?

[oldsmobiledave]

A well balanced drivetrain involves a lot more than just gear selection. You need to consider your engine cid, camshaft specs, converter size & stall, vehicle weight & tire size.

For example a 73-77 car with a mild 455, T400 with a stock converter & a 27" tall tire (255 60 15s) a 3.23 gear is an excellent all around combination. Because of the cid the 3.42s are not needed. More gear means more rpm on the highway unless you go to a taller tire. The gears will help out of the hole but kill you on the highway unless you have tall tires or overdrive.

[zodiacblue442]

I've got a 3.23 in my 455 equipped 73 442. Basically the same setup as oldsmobiledave described. My car has very good performance and is alot of fun to drive. I would think 3.42 would be a little too much gear unless you've got a manual transmission or an overdrive automatic. The engine will be revving more at highway speed and will drink more gas. My brother has a 3.08 in his 70 Cutlass Supreme conv. that's got a 350 ci and TH-350 combo. That car runs like a scared rabbit and still gets an honest 18 to 19 mpg on the highway (when he keeps the a/c off! :D ) He also had a 3.08 rear end in his 455 powered 77 442 and that car was darn fast too.

[msco123]

Guys, thanks for the input. Right now I plan one using the original 350 engine so you think a 3.08 would be best for this motor?

[73431Cutlass]

I have a 425 with a turbo 350 and a 2200 or so stall and 3.73 gears with 26 inch tall tires. At 60 I am turning 3100 rpm or so.... Not a highway car at all but very fun non the less on the street.... I always need back tires for some odd reason! :eek:

[oldsmobiledave]

I have a 425 with a turbo 350 and a 2200 or so stall and 3.73 gears with 26 inch tall tires. At 60 I am turning 3100 rpm or so.... Not a highway car at all but very fun non the less on the street.... I always need back tires for some odd reason! :eek:

With 3.73s you could put a 275 60 15 on that car out back. Your 425 has more than enough low end torque to spin that tire & your highway rpm would go down to a more reasonable rate.

[73431Cutlass]

I used to run those... I run 295/60/15's now, I have it set up so the tires are the same height and the rear are super wide and fill up the fender well..... So I sacrifice the rpm for a good look!

[stews73ho]

I have 3.42s in my 73 HO with a 455. I probably would leave the 3.08s in or go to 3.23s next time. Seems like the rpms are a little high at 65 or 70.

[Joe Pawlowski]

Great choice for all around driving/performance.The 3.23,3.42 or even the 3.55 are great gears as long as you are running stock or larger tires.I have a 3.23 with 255 60 15's and is perfect in my opinion.Dont go any steeper than a 355 though if you want to still cruise at 65 on highway.Joeski

[AGCAWUSN]

I swapped out a 4.11 rear for the 3.42 so I could actually drive it on the highway. Now it bogs down in first, and the screaming burnouts I used to do are out of the question. Should I have done something else to balance it out?

[oldsmobiledave]

[QUOTE=AGCAWUSN;30551]I swapped out a 4.11 rear for the 3.42 so I could actually drive it on the highway. Now it bogs down in first, and the screaming burnouts I used to do are out of the question. Should I have done something else to balance it out?[/QUOTE

3.42 gears are plenty even with the smaller 350 engine. If you can not do a burn out it & it is bogging out of the hole it sounds to me like carb tuning issues.

[AGCAWUSN]

Thanks Dave! That makes a few other issues (hesitation and backfiring)that I am having make more sense.

[Johns 77 442]

The question I have is when you do change out the gears did you also go with a posi unit and which brand of posi is best and how much should all this cost to do at once?
Thanks, John

[pappy86]

I replaced my open 2.41 with a eaton locker and 4.10 in my 1976 with a fairly strong 355, car runs great, my rear tires are 295-50-15 which are almost 27 inches tall. I could do burnouts with either rearend.

[Hurstolds1973]

3.08 & 3.23 's w/posi here with 245 BFG's ... Great all around road and killer burnouts!