Join Date: Feb 2005
Location: Smithfield, VA
Bronco Info: 91 302/E4OD/3.55, F-150 cab
No problemo. Method differs slightly depending on whether it's open or limited slip, so that's the first thing you check. Chock the front wheels and lift both rears off the ground, but leave it in park or in gear. Try to turn one wheel forward. If the other wheel turns backward easily, it's either open or a very worn out lsd. If it won't turn (or takes at least about 30lb-ft minimum to turn), then it's limited slip.
Now, with that out of the way, here's how you check the ratio for each.
Limited slip: Leave both back wheels off the ground, put transmission or transfer case in neutral. Use chalk or a crayon to mark the driveshaft and a spot on the tire. Rotate the back wheel forward, and count the number of full rotations as w. Have a friend count the number of full rotations of the driveshaft, and call that number d. Your gear ratio is d/w. You can get away with only rotating the wheel a couple of times, but the more rotations you count, the more accurate the results. For instance, I usually prefer about w=10, it's big enough to be accurate but makes for easy math. With my 3.55's, that means the driveshaft turns 35 and 1/2 times -- enough turns that you can actually tell the difference between a 3.50 and 3.55 ratio by whether or not the shaft makes that last half turn. If you only turn the wheel twice, you get 7.1 turns on the driveshaft. Can you really eyeball 0.1 turns accurately? If you counted 7 turns on the driveshaft, you'd get 3.50, but it's still "close enough" if you just what a rough idea of what a vehicle has.
The procedure for an open diff is very similar but just as simple. Put the tranny in neutral, but put the opposite wheel down on the ground. When you turn your wheel, since the other wheel is still, the diff will turn half as many times. This throws a factor of 1/2 into the formula. Again, counting w (wheel revolutions) and d (driveshaft revolutions), your gear ratio = 2*d/w. I usually use 20 turns of the wheel when I want to be accurate with an open diff. For my 3.55's, that means 20 turns on the wheel (with the other wheel on the ground) would give me 35 and 1/2 turns on the driveshaft again.
The cliff's notes:
Count w = number of wheel revolutions, d = number of driveshaft revolutions
LSD: Both wheels in the air, ratio = d/w
Open: One wheel on the ground, ratio = 2*d/w