The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Anybody running a big block with th400 and 3.55 gears?

*Fitz* · 04-20-2012, 11:16 AM

[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.

04-20-2012, 11:16 AM	#7
Fitz Account Suspended Join Date: Jan 2007 Location: Tucson Posts: 2,183	Re: Anybody running a big block with th400 and 3.55 gears? [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.