My '77 F150 has a 3/4ton rearend with no tag on the diff. I need to know the gear ratio because I'm replacing the 1/2ton frontend with another 3/4ton front. I had the cover off once but didn't count the teeth. I do remember seeing a number 41 stamped on the ring gear. Today, I jacked it up and spun the driver's side tire while counting the turns of the driveshaft. It turned approx. 1 7/8 times for one turn of the tire. I think the rearend is limited slip because the tires will turn in opposite directions when spun free while jacked up. I once heard that that's an indication of limited slip. I also heard that when calculating your gear ratio in this manner on a limited slip axle, you should double the number of rotations. 1 7/8 X 2 would be 3 5/8, correct? Which would equal approx. 3.54:1 gears, correct? If so, that would be great because, according to the tag on the 3/4ton frontend, its a 3.54:1 gear.
Would somebody please tell me if any of this makes sense? There are several reasons for the frontend swap. The 1/2ton's ring gear is busted, I think due to the different gears between front and rear. Another is just that it would be nice to have matching axles. Also, the swap will change the front hubs from five lug to eight lug which will match the rear. That'll make tire rotations easier. Any help, advice, direction would be appreciated.

 

well...your rear is not a limited slip if the wheels rotate oppisite directions. Hate to break it to you but you have an open diff....

 

You pretty much got everthing backwards

Open diff with both wheels jacked up and trans in gear, wheels turn opposite directions (same with wore out Trac-Loc).

L/S will be a bear to turn that way (takes 250 ft lbs or so to over ride the clutchs).

With an open diff, one tire on ground, the tire in the air will turn twice what it would if both wheels were turning. With a 4:1 rear gear, the tire would turn once for every two driveshaft revolutions.

Take the rear cover off and count the teeth.

 

great Thanks for the info. I was hoping to avoid taking the cover off again.