Ok I have a 70 Chevelle with a mildly built 350 pushing a Borg Warner T-56 6 speed. The tranny gear ratios are as follows:
#1 2.66:1 #2 1.78:1 #3 1.30:1 #4 1.00:1 #5 0.74:1 #6 0.50:1
Currently I have a 12 bolt with an Eaton Posi and Moser axles using 3.55 gears. With these gears I never touch 6th, and rarely 5th. Off the line I can easily push past 40 mph in first. The problem is that I have very little low end power, so I am planning on a change, but was curious what ratio you guys would think best. I am looking between a 4.10 or a 4.56 and here are how the numbers come up (with a 26.60 inch tall tire)
4.10
Gear Ratio Final MPH@rpm RPM@70

1 2.66 10.91 43.5 9649
2 1.78 7.30 65.0 6457
3 1.30 5.33 89.1 4716
4 1.00 4.10 115.8 3627
5 0.74 3.07 154.4 2721
6 0.50 2.05 231.6 1814
R 2.90 11.89 39.9 10519

4.56
Gear Ratio Final MPH@rpm RPM@70

1 2.66 12.13 39.1 10731
2 1.78 8.12 58.5 7181
3 1.30 5.93 80.1 5245
4 1.00 4.56 104.1 4034
5 0.74 3.42 138.8 3026
6 0.50 2.28 208.2 2017
R 2.90 13.22 35.9 11700
I am thinking that the 4.56 look the best, since I can easily still use 6th on the freeway, being only at 2017RPM at 70mph. But experience may say different, so this is why I rely on you guys for your input! Thanks in advance!

4:10's will do your set-up justice IMO.

Nice dilemma. Hey, how about sharing some installation details? What did you do for the clutch? Where did the shifter location end up?
Personally. I like to lug 'em for mileage. I'll probably go with 3.31s, just because that's what's already in the irs I plan to add. But, for causing general mayhem, sounds like the gears you like should do fine.

Nice dilemma. Hey, how about sharing some installation details? What did you do for the clutch? Where did the shifter location end up?
Personally. I like to lug 'em for mileage. I'll probably go with 3.31s, just because that's what's already in the irs I plan to add. But, for causing general mayhem, sounds like the gears you like should do fine.

The T56 and 4:10's will give the best of boths worlds. This is one of the most versatile trannys ever made for a factory vehicle. The rearend gearing will give a gut-wrenching hole-shot and the 5th and 6th gear of the T56 will give you nice numbers on mileage and RPM's.

Installation details... If only I could remember! I first put it in around 1999, so it has been a few years! I did haver to use a conversion flywheel to use the clutch, and the hydraulics were a little bit of an issue to fabricate since the car was originally an automatic, so I had to switch the pedals and everything over. If you need info IM me and I can tell you all I remember... So for the gears, I rarely am on the freeway, mostly cruising around town, which is why I want more low end grunt, and why I was thinking the 4.56 would be best.

Installation details... If only I could remember! I first put it in around 1999, so it has been a few years! I did haver to use a conversion flywheel to use the clutch, and the hydraulics were a little bit of an issue to fabricate since the car was originally an automatic, so I had to switch the pedals and everything over. If you need info IM me and I can tell you all I remember... So for the gears, I rarely am on the freeway, mostly cruising around town, which is why I want more low end grunt, and why I was thinking the 4.56 would be best.
If a killer holeshot and low-end power is your goal and you don't care about losing a few mph on your topend than definitely go with the 4:56's

I have the same T56 ratios with the .50:1 6th gear. I have approx. 27" dia. rear tires & a 3.73 posi 12 bolt. I love it! It comes up through the gears driving/cruising around town using 4th & even 5th. It does the 2-lane driving 50-60+ mph using 5th nicely, & when I want to run the interstate, I use 6th around 68+mph. 70mph is around 1800-1900 rpm. I have approx. 350hp 396 with a Edelbrock 750 & get 21mpg running the interstate on trips.

I know others running 4.10's & aren't greatly different driveability from mine. Since I don't drag race the car, I was looking to get a little farther before shifting to 2nd around town. My typical around town shifts are around 2500rpm.

Here's a chart that you can play with on rpms/tire dia.'s/shift points, etc.:

http://www.geocities.com/z_design_studio/transmission_z28_6.html

Have fun as you proceed.

Odie

Might want to run this by a driveshaft shop??

You might over-speed your driveshaft in OD b/c it may just break and get thrown out at speed!

pdq67

There is a 4.30:1 rear gearset available for the GM 12 bolt. Run your numbers with this ratio. This would be my choice with a 2.66:1 1st T56.

You also need to know what your camshaft power band is. This will help determine what ratio will and won't work as your final drive.