Hey all, just wondering how to tell what size rear end i got in my 70 dart. It's a 318 with a 904. Not really too knowledgeable about differential gears. Been thinking about upgrading and wanna know what i'll need.

 

Put a mark on the tire and driveshaft, move the car so the tire has one rotation, count the number of times the driveshaft turns and divide. If you open it up, count the teeth on the ring then the pinion and divide.

 

you prob have a 7 1/4 with 2.76 open

 

It should have a 8 3/4 There is a slim possibility it is a 7 1/4

 

I doubt that VERY much. More likely an 8 1/4, BUT THERE is only one way to find out, LOOK:

To figure out the ratio, you FIRST must find out if you have an "open" or "non" Sure Grip (limited slip) or whether you DO have a Sure Grip

So jack up JUST one rear wheel, with the front wheels blocked. Release the parking brake, and put the car in neutral. See if you can turn the rear tire. If so, Turn the tire TWO exact full turns while counting driveshaft turns. A 2.76 will turn the shaft 2 3/4 turns, 3.55 3 1/2 plus a tiny, 3.23 3 1/4 turns, etc etc

If you can NOT turn the tire, you have a Sure Grip or "limited slip". Now you'll have to jack up BOTH rear tires, but now, turn one of them ONLY ONE full turn and count shaft turns. The other tire should follow exactly as you turn the first

 

78doubleR was right 7 1/4. Thanks. So now any ideas/suggestions about upgrading? This is the original factory setup.

 

dam i wish i had this pic two days ago spent 6 hours walking threw a 74 & older bone yard and could not figure out if it was a dana 60 or a 9 1/4 found like 15 8 3/4 & 3 or 4 9 1/4 looks like time to go back ! bone yards rule if you can find them

 

knew it!!
ditch it for a 8 1/4 or 8 3/4 with 3.23 or 3.55 sure-grip