Help determining rear end gear ratio.

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May 26, 2002 | 10:55 PM

unioncreek

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Help determining rear end gear ratio.

I have just recently finished rebuilding my 390, I added a tach last week. At 60 mph tack reads 2750, I thought I had a 3.50 rear end. The formula I have says at that rpm with 27.75 inch tires that I have a 3.75. I jacked it up tonight and marked the drive shaft and tire and turned the tire one turn. The drive shaft turned 1 3/4 times, if I had a 3.75 shouldn't it have turned 3.75? I am probably not correct in my assumption so could someone correct me.

I would like to increase gas mileage and be turning about 2200 rpm's any comment on wanting to do this and the gear ratio I should use.

Thanks,
Bob

May 26, 2002 | 11:01 PM

Ratsmoker

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From: Missouri

Help determining rear end gear ratio.

If you have an automatic tranny its kind of hard to say what gear you have due to slippage.

May 27, 2002 | 05:40 AM

jowilker

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Help determining rear end gear ratio.

Bob, I can never remember to lift one wheel or both so I would suggest using your marks and rolling the truck along a very level spot of ground or floor. Have another adult with you to do the rolling while you watch and to be there just in case all doesn't go well.

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May 27, 2002 | 10:55 AM

Ratsmoker

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Help determining rear end gear ratio.

Memory is the first thing to go John!

It's one wheel off the ground for a conventional and both for a posi rear.

May 28, 2002 | 07:28 PM

red68ranger

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Help determining rear end gear ratio.

With an open rear end and one wheel off the ground the turns ratio you observe is one-half the rear end ratio. 1.75 turns is the figure you should have gotten with a 3.50 rear end.

Dan Blackwell
(turned a wheel 18 times and saw the shaft turn 27--3.00 is twice 1.5)

May 29, 2002 | 05:45 AM

jowilker

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From: Creedmoor, North Carolina

Help determining rear end gear ratio.

I know that's right Sean, but rolling on the floor or very level ground should work for both types, right?

May 29, 2002 | 06:53 AM

Ratsmoker

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Help determining rear end gear ratio.

Yep you are right John. Rolling it along the ground with the help of a buddy like you said is probably the easiest way. I do believe that one wheel off the ground will give you a 1:1 ratio. I have been wrong before but I'm pretty sure. I'll check it out for myself as I am currently changing out my diff gears within the next couple days.