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Discussion Starter #1
I am trying to determine what this is. I found 13 listed as a 2.75 ratio for 77 to 79 but I can't find a definition of the X. Would that mean no front axle? It is a 2 wheel drive.

If this is what I really have its no wonder the thing will run 50 in first gear.
 

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You're leaving out too much info.

What year and model vehicle are you talking about?

I put 2.75's into my 64 T-bird years ago, they came from 71 Galaxie 390 car. My bird was able to do almost 160 MPH with those gears and a stock engine...

Anywhoo, you mentioned No front axle, NOW you're talking Truck. But with a truck data plate the Axle Gear Ratio covers both front and rear.
 

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I am trying to determine what this is. I found 13 listed as a 2.75 ratio for 77 to 79 but I can't find a definition of the X. Would that mean no front axle? It is a 2 wheel drive.

If this is what I really have its no wonder the thing will run 50 in first gear.
Not sure but I think I read somewhere when I was looking up Diff. info that when there is a letter after the number it means its posi or trac loc????
Latter when I get time I will try to find the site I seen it.
 

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Discussion Starter #4 (Edited)
This is my 79 F150 custom explorer, 302, C4, truck, 2 wheel drive. I am fairly confident that it is the original rear end because the truck only had 115,000 miles on it when I got it I'm going to go out in a minute and see if I can get the numbers off the third member tag. The table on the fordification.net site says a code 13 is a 2.75 gear and the letter is for the front axle. Well duh, no front axle on this truck. If this is a traction lock it must have given up some time ago because it won't lay down both tires to the asphault.

Presuming it really is a 2.75, I'm getting another 9 inch for my Yota Cleveland drag truck and it is supposed to have 3.25's. I will be running 4.56's in the truck so I'm planning to pull the third member and put in my truck to help me pull my car hauler. 2.75 to 3.25 is an 18 percent lower gear. Does that sound like enough gear that it will help keep me from blowing my C4 again? Granted, I had a hard part fail that probably wasn't from towing but Jay said that it showed some heat damage.

After doing some more reading I am now a bit concerned about the spline count as well. The other 9 inch is off an earlier Ford truck, 71-2 I think. I'm trying to figure out how I can tell if both these will have the same axle spline count and what is the spline count on my current rear end. Jeez, why is simple stuff so complicated, LOL.
 

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Discussion Starter #6
My tag sayahs itsa WDM CJ 9 BD,,,,,,,,, 2.75 9 inch, non locking, 28 spline so I need to check the other rear now when I can. It is coming off a plain jane small block early 70's 2wd half ton pickup as well so I "SHOULD" be ok, LOL(nervously laughing). I think the worst case is going to be if I have to swap the rear yoke but that is not a biggie.
 
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