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Rear end gearing
#1401494 Mon Mar 15 2021 07:31 PM
Joined: Feb 2020
Posts: 21
B
Bodini Offline OP
'Bolter
Would appreciate some thoughts on rear axle gears. Current rear gears are 3.55 with a Muncie 420 behind a mildly warmed over 235. Taking off in 2nd (3.57) winds out pretty quick before shifting. I just picked up a TKO600 with the 2.87 first gear and the .82 OD that's gonna run back into a Camaro 10 bolt. Should I stick with the 3.55's or would 3.42's be ok. I don't want to ask too much from the engine and clutch taking off in first gear........or am I just making a big deal out of nothing?

Re: Rear end gearing
Bodini #1401497 Mon Mar 15 2021 07:40 PM
Joined: Jun 2004
Posts: 8,451
W
'Bolter
I think the 3:55 would be just right for that.
George


They say money can't buy happiness. It can buy old Chevy trucks though. Same thing.

1972 Chevy c10 Cheyenne Super
Re: Rear end gearing
Bodini #1401513 Mon Mar 15 2021 10:43 PM
Joined: Jun 2011
Posts: 4,274
E
'Bolter
The difference between 3.55 and 3.42 is so small, one probably couldn’t tell the difference.

Stick with what is in there.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
Re: Rear end gearing
Bodini #1401521 Tue Mar 16 2021 12:06 AM
Joined: Jul 2000
Posts: 1,335
G
Insomniac
Originally Posted by Bodini in the "need for speed" thread
I have a 51 3100 fully restored. The engine is a 235 and the numbers say it's a 58-62 block. I have an SM420 bolted behind it, torque tube rear with the 3.55 setup. The truck has 15 inch wheels with P225x75 tires in the rear. She runs like a dream....up to 50 mph. It would be nice to at least go 55-60 if I get on the highway to go somewhere. If I bump up to 16 inch wheels and tires is that gonna give me 5-10 mph more? Should I go with a "regular" 4 speed, open rear end with lower gears? If so, what easily swapped 4 speed and rear end would you graduates of Truckology recommend? Thanks for any and all ideas.
Bodini

Do you have a Camaro rear end or are you shopping for one? Camaro rear ends came with a variety of gears including 3.55 and 3.42 depending on the year, engine and transmission. Camaro web sites have info on how to ID a rear end.


Gord
----
1954 1/2 ton 235 4 speed
Re: Rear end gearing
Bodini #1401542 Tue Mar 16 2021 02:16 AM
Joined: Mar 2004
Posts: 5,276
M
'Bolter
Always tell us what year and model truck you're working on.

Thanks,

Mike B smile

Re: Rear end gearing
Bodini #1401552 Tue Mar 16 2021 03:28 AM
Joined: Mar 2014
Posts: 2,560
J
'Bolter
The 3.55 with a 2.87 first will give you a starting line ratio of 10.18. The 3.42 and a 2.87 first is a slr of 9.81. If you're planning on using it as a pickup, I would leave the 3.55 in there. 10.18 is ok for a 235 but honestly speaking that ratio is just barely ok if you're hauling anything heavier than a sheet of plywood and a few 2 x 4s. GM wanted a starting line ratio of 12.08 for the 235 in a 1/2 ton pickup. I would not consider any ratio below 10 unless I had a strong V8 and was setting up to drag race and even then not with a standard transmission. Good luck.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Rear end gearing
Bodini #1401564 Tue Mar 16 2021 04:05 AM
Joined: Feb 2020
Posts: 21
B
Bodini Offline OP
'Bolter
Thank you gentlemen. The truck is a 1951 3 window 3100. I picked up the 1978 Camaro rear from a salvage yard and plan on having it rebuilt so no worries in the future. Looks like I'll stick with a 3.55 setup. I don't plan on putting anything in the bed except for a couple lawn chairs when I go to a show.

Re: Rear end gearing
Bodini #1401614 Tue Mar 16 2021 04:17 PM
Joined: Mar 2019
Posts: 117
T
'Bolter
I agree that in your situation the difference between 3.55 and 3.42 is minimal. That is less that 4% difference in driveshaft speed at any road speed. You can get that much difference just due to the different diameters of the various brands for tires in the same nominal size. I doubt if anyone driving a vehicle could tell the difference.

So take the easier/lower cost/less risk route!


Rusty

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