Rear End Gear Ratio Question
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 1,146
Likes: 0
From: Milford NH
Rear End Gear Ratio Question
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.
Axle Ratio = (RPM x Tire Diameter ) / (MPH x 336)
I am at 2500 RPM when I cruise 60 MPH in four gear.
Team Owner
Joined: Aug 1999
Posts: 28,723
Likes: 4
From: Fort Mill, SC via NJ
Re: Rear End Gear Ratio Question (69stingray)
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]
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]
Racer
Joined: Oct 2002
Posts: 393
Likes: 0
From: texas city texas
Re: Rear End Gear Ratio Question (69stingray)
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:
Drifting
Joined: Feb 1999
Posts: 1,260
Likes: 5
From: Brandon FL
Re: Rear End Gear Ratio Question (mp77)
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.
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.
Melting Slicks
Joined: May 1999
Posts: 2,897
Likes: 0
From: Moon Township PA
Re: Rear End Gear Ratio Question (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.
Axle Ratio = (RPM x Tire Diameter ) / (MPH x 336)
I am at 2500 RPM when I cruise 60 MPH in four gear.
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
Safety Car
Joined: Feb 2002
Posts: 3,556
Likes: 0
From: Seattle WA
Re: Rear End Gear Ratio Question (Markm10431)
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:
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:
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Le Mans Master
Joined: Mar 2002
Posts: 6,217
Likes: 1
From: End of the Ike IL
Cruise-In V Veteran
Cruise-In VI Veteran
Re: Rear End Gear Ratio Question (Markm10431)
you have an '82 4 speed auto in a '68?
[Modified by BlueL36, 11:56 PM 10/24/2002]
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 1,146
Likes: 0
From: Milford NH
Re: Rear End Gear Ratio Question (69er)
I have 3:55's and at 70MPH I'm at 3100 RPM.
How is cursing at 3100 RPM? Is 3100 too high or do-able?
Team Owner
Joined: Jun 2001
Posts: 43,136
Likes: 155
From: Thomson Georgia
Re: Rear End Gear Ratio Question (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.
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.
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
Le Mans Master
Joined: Oct 2001
Posts: 5,145
Likes: 125
From: The Motor City
Re: Rear End Gear Ratio Question (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.
You sound higher than the 3:10(3:08)you think you have.
You seem to have a 3:36.
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]
