Rear end question
Thread Starter
Melting Slicks
Joined: Feb 2006
Posts: 2,092
Likes: 5
From: Fayetteville Arkansas
Rear end question
Took the ole 72 for a spin around the block to get gas and man was that fun! It's so nice to actually get to drive it a bit after all I've done to it over the last 5 years. I'm supposed to have a 3:55 rear end gearing. At least that's what the codes show on the underside of the pumpkin. But it sure does seem that it is geared lower than that. What the general guestimate equation where something like get going in third gear and get it to a certain MPH and then see what the RPMs are? I've read it here somewhere an can't remember now. Anyone have a clue what I'm talking about?
Thanks
Sully
Yes I know I haven't got pics up yet but I still have to finish the alignment of front bumper, a bit more work on fitting of interior panels, and other odds and ends then ill get some pics up.
By the way it's a 72 coupe with stock 350 and 4 spd
Thanks
Sully
Yes I know I haven't got pics up yet but I still have to finish the alignment of front bumper, a bit more work on fitting of interior panels, and other odds and ends then ill get some pics up.
By the way it's a 72 coupe with stock 350 and 4 spd
Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
there are gear ratio calculators on this forum and elsewhere on the web. if it were me, i'd jack up the rear end (both wheels off the ground), car in neutral, make a chalk mark on the bottom of your tire and on your driveshaft. spin the tire one full revolution and count the number of driveshaft revolutions. 3 1/2 revolutions = 3.55
Thread Starter
Melting Slicks
Joined: Feb 2006
Posts: 2,092
Likes: 5
From: Fayetteville Arkansas
Looks like I'll be getting out the kids sidewalk chalk. 
I do hope it's a 3:55 and not lower Bc I am wanting to drive this to work and back during nice weather which is a 30 min highway drive.
I do hope it's a 3:55 and not lower Bc I am wanting to drive this to work and back during nice weather which is a 30 min highway drive.
Burning Brakes
Joined: Jul 2008
Posts: 914
Likes: 10
From: Brooklet Ga.
there are gear ratio calculators on this forum and elsewhere on the web. if it were me, i'd jack up the rear end (both wheels off the ground), car in neutral, make a chalk mark on the bottom of your tire and on your driveshaft. spin the tire one full revolution and count the number of driveshaft revolutions. 3 1/2 revolutions = 3.55
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
The 4.11 will allow the car to accelerate faster off the line at the expense of gas mileage and higher revs at any given speed.
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
Took the ole 72 for a spin around the block to get gas and man was that fun! It's so nice to actually get to drive it a bit after all I've done to it over the last 5 years. I'm supposed to have a 3:55 rear end gearing. At least that's what the codes show on the underside of the pumpkin. But it sure does seem that it is geared lower than that. What the general guestimate equation where something like get going in third gear and get it to a certain MPH and then see what the RPMs are? I've read it here somewhere an can't remember now. Anyone have a clue what I'm talking about?
Thanks
Sully
Yes I know I haven't got pics up yet but I still have to finish the alignment of front bumper, a bit more work on fitting of interior panels, and other odds and ends then ill get some pics up.
By the way it's a 72 coupe with stock 350 and 4 spd
Thanks
Sully
Yes I know I haven't got pics up yet but I still have to finish the alignment of front bumper, a bit more work on fitting of interior panels, and other odds and ends then ill get some pics up.
By the way it's a 72 coupe with stock 350 and 4 spd
If you have 3.36 gears, about 60 MPH at 2500 RPM. If you have 4.11 gears, YOU WILL KNOW without even looking at the speedo.
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Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
Yes. With a 4.11 the engine turns 4.11 times to turn the wheel one revolution. With a 3.55 then engine turns 3.55 times to turn the wheel one revolution.
The 4.11 will allow the car to accelerate faster off the line at the expense of gas mileage and higher revs at any given speed.
The 4.11 will allow the car to accelerate faster off the line at the expense of gas mileage and higher revs at any given speed.
Just to clarify, this is more accurate if you substitute "driveshaft" for "engine." Engine speed only matches driveshaft speed in 3rd gear (1:1) on a THM400/350 or 4th gear on a Muncie. The numerical ratio is the number of teeth on the ring gear divided by the number of teeth on the pinion gear.
My 3.73s have 41 ring/11 pinion = 3.727
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
If you have 27 inch tires.. YES or very very close. If you have 26 inch tires.. subtract about 2MPH. I could be wrong but you might have 3.36 gears instead 3.55. Which is not a huge difference in performance but you will have higher MPH at 2500. about 60MPH instead 56MPH.
Last edited by jackwabbit703; May 16, 2013 at 01:19 PM.
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia