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PostPosted: September 2, 2017, 12:47 pm 
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Location: iowa, usa
How do I figure out the gear ratio of a rear differential that has the tag removed? The one I'm looking at (Craigslist) is out of the car but is the complete axle end to end. The seller says he's sure it's a 31 spline and either 2.73 (I assume he means 2.75) or 3.00 gears. Thing is it's a bit of a drive to get it so I want to know what it is before I leave. Am I correct in thinking I can turn the differential one time around and that should give me 2.75 turns at the wheel hub? Is there anything written on the axle case (inside or out) that will identify the ratio?
Seller states it's out of a 75 F100 which from looking at the "FORDification" rear axle ID website leads me to believe it's probably a 3.00 geared unit not a 2.75.
This is actually going in my 69 Galaxie but I figure the information is useful and kinda relates to our trucks as well.

Thanks for any help.
Scott.


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PostPosted: September 2, 2017, 3:51 pm 
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If it is an open diff you can put it in neutral, parking brake off, chock the wheels, and jack one of the rear tires up off the ground. Mark the driveshaft yoke. Rotate the back tire that is off the ground and count how many times it takes for the driveshaft to go around once. Divide that number by 2 and you will know the approximate ratio of the diff.

Dan

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PostPosted: September 2, 2017, 9:40 pm 
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If the axle spline is important to you, then you should pull an axle to check. You can't believe anything seller's say.

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PostPosted: September 3, 2017, 8:10 am 
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Thanks guys for the information looks like rotating it should get me close. And yep I will definitely check the splines myself.

Scott.


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PostPosted: September 3, 2017, 1:03 pm 
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This is how I see it. If your going to pull an axle to check spline, why not just pull the chunk out and count teeth?


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PostPosted: September 3, 2017, 6:39 pm 
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If you pull the chunk the ratio will be stamped on the side of the ring gear :2cents:

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PostPosted: September 7, 2017, 7:00 am 
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Yeah I'll be pulling the axles and chunk both because like you say ya never really know till you look, and thanks for the reference to checking what's stamped on the ring gear I didn't know that.

Thanks guys.
Scott.


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PostPosted: September 8, 2017, 8:28 am 
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There is also the chance the original axle tag is still attached which should provide all the information.


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