69stingray

Using the following formula, I have determined my rear end gear ratio to be 3.10, does that sound right? If so, how much of a project is it to incease it to 3.55, what improvment in accerlation would I see?

Axle Ratio = (RPM x Tire Diameter ) / (MPH x 336)

I am at 2500 RPM when I cruise 60 MPH in four gear.

69er

I have 3:55's and at 70MPH I'm at 3100 RPM.
You sound higher than the 3:10(3:08)you think you have.
You seem to have a 3:36.




[Modified by 69er, 6:14 PM 10/24/2002]

mp77

Sounds a little strange. I hear a lot of people saying they have 3.08 to 1. Anyway, I have never swapped gears. I would have someone do it. But thats me, cause I am lost on any suspension work :confused: except for shocks and bushings. :yesnod:

joevette57

If yo get the entire pumpkin, swapping a rear end is a day job by yourself, about 4 hours with a helper. (Assuming everything comes apart properly)
I swapped the 3.70 for a 3.08 in my 70. my myself started at around 9am, finished around 2pm, and I've done a few of these. Biggest pain is dropping the rear crossmember. Can be a bear.

GaryS

3.10 is so close to 3.08's that one has to assume that you have the latter. I just helped Glen242 change out his differential due to an internal failure. Count on three hours out and another three back in. It is pretty much grunt work to R&R a differential, not requiring lots of finese. I would suggest that if you go this route, you get an Assembly Instruction Manual and the GM Service Manual for your car. They give SPECIFIC instructions on removing the components so as to not damage anything or anyone.

Also, before you start, get a good estimate on the cost of the gear change and then add in new u-joints, new spring bolts, new front differential mount, and maybe some strut rod bushings. Most of this you will want to do as long as the differential is out so you don't have to do it later. You will also want to have some time to clean up that area since you don't want to go and put a shiny new differential in your rusty back end.

Don't forget that there may be added costs if, for instance you need new shocks, since there is that cost as well as the tool to take the shocks off.

Gary
btw, a good estimator for differential ratios can be found at http://www.idavette.net/tech/ratioc.htm

BlueL36

I have a TH400 w/ 3.08 rearend and its 2500 rpm at 60 mph.

Markm10431

you have an '82 4 speed auto in a '68? who would do that?? I'd kill to have a manual tranny

'79ProwlerOrange

there werent any manual 82s at all so unless it was a camaro 4-speed, he couldnt do it. :confused:

3.08s sounds right. i had 3.55s and did 60 at 3000 rpm. with 4.11s i do 60 at about 3700 rpm :eek:

BlueL36

Nope, it's the 3 spd AT that came with the car. Purrs like a kitten.


[Modified by BlueL36, 11:56 PM 10/24/2002]

69stingray

Thanks guys. I think I must have the 3.08, it is the base model.

How is cursing at 3100 RPM? Is 3100 too high or do-able?

Alwyn678

Except I went from 3:08's to 3:73's....Cause I love that launch and putll through the midrange..where my cam comes in

Scooter70

I think I'd agree with 69er, depending on your tire size. I have an Excel spreadsheet where you input your tire size, trans ratios, and rear end ratio, and it will calculate speed at a given rpm, or rpm at a given speed.

With 255/60/15s at 60mph,
3.08=2297 rpm
3.36=2505
3.55=2647
3.70=2759
4.11=3065

I can send the Excel sheet to anyone if they want it.

**Edit: better yet, right click->Save As..... http://matt.undiagnosed.org/rear_gear_calc.xls

Zach, the best way to figure out what gear you have is to jack up the car and spin the driveshaft 360*. Count the revolutions of the tire.


[Modified by Scooter70, 4:28 PM 10/25/2002]

Bob Turner

Scooter70-

Can I get that spreadsheet? bobturner@richdad.com

Thanks,

Bob