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Re: rear end swap
Pastor James #1404110 Wed Mar 31 2021 10:44 PM
Joined: Feb 2019
Posts: 2,074
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Originally Posted by Pastor James
The rear end on this truck has been changed whats the easy why to discover what i have?
You can jack up one wheel and count the revolutions of the drive shaft and get a pretty good idea (not exact) of what you have.

In order to do this, chock the front wheels, jack up one rear wheel, release the emergency brake and place the transmission in neutral. Mark the tire so you can count one revolution, then mark the drive shaft also so you can count the revolutions. With a helper, turn the driveshaft counting the revolutions, until the tire turns one complete turn.

Now to calculate the ratio, divide the revolutions of the driveshaft by two. As an example, if the driveshaft went around 8-1/4 times, divide 8.25 (converted to decimals) by 2 and you get 4.125. That would most likely be 4.11.

Disclaimer: If you have a posi-traction rear, you will need to jack up both wheels and you don’t need to divide it by 2. The actual revolutions of the drive shaft needed to turn the tire one turn, is the gear ratio.

If you need to know the exact ratio, you will have to pull the rear cover. Some gear sets have the ratio punched into the edge of the gear. If not, you can count the gear teeth and divide the larger number by the smaller number.

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Last edited by Phak1; Wed Mar 31 2021 10:57 PM.


1952 Chevrolet 3100
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Re: rear end swap
Pastor James #1404185 Thu Apr 01 2021 12:04 PM
Joined: Nov 1995
Posts: 5,143
Unrepentant VW Lover
Pastor J -- No, that brake tech tip is still in someone's head waiting to be written ... wink But now that I have smacked them all on the hands with my ruler, that's not very likely to happen ... at least in my lifetime.

Ahem. Where were we? Ah yes. When you say that the rear end has been "changed" ... what do you mean? The whole axle carrier assembly has been changed? Just the gearing has been changed?

If the whole shebang has been replaced with something newer ... well, we'll need to see some pictures of it (front and back of the differential, as well as the lug pattern) to identify it and we can go from there. But first, follow Phak's instructions on determining the ratio -- that's the first step here -- determine the "as is" before formulating a plan for the "to be."

But if we may back up a bit here ... have you driven this truck yet? If it has a different rear end ... have you established that it is, in fact, "slow"? Just making sure we're aren't jumping to possibly erroneous conclusions here ... smile

Nattering Nabob of Reality


"Of all tyrannies a tyranny sincerely exercised for the good of its victims may be the most oppressive."
— C. S. Lewis

'49 Chevrolet 3804

Re: rear end swap
Pastor James #1404298 Thu Apr 01 2021 09:58 PM
Joined: Mar 2017
Posts: 261
Go back on what Johns link it is correct. It tells you exactly what you need to find and complete the task.

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