Anybody running a big block with th400 and 3.55 gears?

[Racie]

I'm about to do some upgrades to my rearend in my 68'. Right now I have an open 3.73 setup. I'm cruising on the highway with normal flow of traffic at 3500 rpms. This is really not a problem besides drinking up the gas lol...but if I'm going to change to a limited slip set up I might as well change gears if I'm going to do it while I'm in there. I've been thinking about running 3.55 limited slip or 3.42's what you guys think and list you mods if you could and what rpms you are seeing.

Here's what I have...

1968 SWB
502 H.O. Crate Engine
Minor head Work still has 2.19/1.88 valves
Bumped the Compression up a bit over 9.5:1
Howard's Hyd Roller cam-Duration:235/241(ADV 288/294)Lift:618/618 LSA:112
Air Gap Intake with Holley 950HP
DUI ignitionHooker Comp Headers 2" primaries 3.5 collector3" Exhaust
Race Built Stage 3 TH400 2800 stall
3.73 gears open carrier

Loving the way my truck is performing but the open rear is killing me!

[extd56]

I would run a 3.08 or even a 2.73 to let that big block live a long and healthy life. The 3.55 will not help much to lower the RPM's. I have a 402 in my 56 Chevy truck with a 400 trans and a 2.73 rear end from a 79 Seville and it works great. You could also have a 700R4 built up for the big block and it would work the best. I have a 4L80 in my 68 crew cab with a 454 and 3.73 gears which works great but also have 35" tires on it. I am running about 2000 RPM at 70 miles an hour and getting about 12 miles to the gallon with a lifted truck.

[CC69Rat]

DIA of your rear tires?

[71 super]

I swapped the 3.08 gears in my 71 to 3.55 and not impressed, wish i had gone with 3.73

[70cst]

Originally Posted by 71 super

I swapped the 3.08 gears in my 71 to 3.55 and not impressed, wish i had gone with 3.73

3:73 That's the combo I have and its a good match.

[SS Tim]

3.07 was stock for BB powered C/10 trucks. My suburban would get 17mpg freeway @65 on L78 tires. That was a pretty much stock but tuned engine with a good air cleaner. With a 502 I'd imagine you are doing a lot better than the 402/210hp. So if you want mileage step up to a 2.73 or consider an o/d trans. swap or a Gear Vendors O/D.

[Fitz]

[QUOTE=Racie;5324138]I'm about to do some upgrades to my rearend in my 68'. Right now I have an open 3.73 setup. I'm cruising on the highway with normal flow of traffic at 3500 rpms.QUOTE]


I had the same problem but went a different way as I really wanted to keep the high speed starts but still get better mileage and less noise on the freeway.
My Truck is running a 572 BB with a TH 400 trans giving the following ratios
1st 2.48
2nd 1.48
3rd 1.0
The diff is a 4.10:1 posi coupled to a gear vendors 0.7:1 or 1:1 overdrive
The tires I use are 285/60R17's with a 30.46 diameter which gives final drive ratios of 5.94 in OD or 8.94 @ 1:1.
that's 2340 rpm @70 mph in OD or 3340 RPM @ 1:1

Much quieter in OD but i can still burn the rear tires all the way through first and second.

[SS Tim]

Originally Posted by Fitz

I had the same problem but went a different way as I really wanted to keep the high speed starts but still get better mileage and less noise on the freeway.
My Truck is running a 572 BB with a TH 400 trans giving the following ratios
1st 2.48
2nd 1.48
3rd 1.0
The diff is a 4.10:1 posi coupled to a gear vendors 0.7:1 or 1:1 overdrive
The tires I use are 285/60R17's with a 30.46 diameter which gives final drive ratios of 5.94 in OD or 8.94 @ 1:1.
that's 2340 rpm @70 mph in OD or 3340 RPM @ 1:1

Much quieter in OD but i can still burn the rear tires all the way through first and second.

Exactly what I was trying to point out. Here is GV's T400 chart to show the effective ratios.

[Racie]

[quote=Fitz;5325768]

Originally Posted by Racie

I'm about to do some upgrades to my rearend in my 68'. Right now I have an open 3.73 setup. I'm cruising on the highway with normal flow of traffic at 3500 rpms.QUOTE]


I had the same problem but went a different way as I really wanted to keep the high speed starts but still get better mileage and less noise on the freeway.
My Truck is running a 572 BB with a TH 400 trans giving the following ratios
1st 2.48
2nd 1.48
3rd 1.0
The diff is a 4.10:1 posi coupled to a gear vendors 0.7:1 or 1:1 overdrive
The tires I use are 285/60R17's with a 30.46 diameter which gives final drive ratios of 5.94 in OD or 8.94 @ 1:1.
that's 2340 rpm @70 mph in OD or 3340 RPM @ 1:1

Much quieter in OD but i can still burn the rear tires all the way through first and second.

Thanks that's exactly what I'm wanting. I think a few misundertsood I'm not wanting to get this dream of having good gas mileage. My 502 was built to pull 6500 all day long but going down the highway at 3800 and 4000 rpms is not great. I want to bring that rpm down some I should have enough torque to cheat and get out the hole I just want to find a compromise with out killing my perfectly matched set up. The truck is a blast to drive and runs 10 times better with the built th400 than the stage 3 crap 700r4 that it completely just murdered. I'm approaching 550 horse and 550 torq numbers and that 700r4 was not up to the job and I'm sure I'm under estimating those numbers because it cam with 502hp and it's night and day after I rebuilt it. I have a 480LE in the garage but I'm saving it for another build. If my truck was driven more than 3 or 4 times a month on the highway I would probably install it but I just can't spending that money on a swap and a nice $1300 computer and hardly drive the truck. Lol...but who are we kidding I'm gonna spend it somewhere! Thanks for the advice...
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