Need help ID'ing my rear end gears

[sandman]

1972 F-250 Sport Custom. C-6 Auto trans, 390 engine.

I have inspected the rear end and there is no tag anywhere on the bolts.

I need to see if I can find out the gear ratio because I would like to change to a lower gear for gas mileage since I dont ever pull anything with the truck. I just want to make sure I dont already have a set of low gears inside. ALthough I dont believe I do. Truck revs alot at highway speeds.

I have only owned this truck for 2 months now and would appreciate any help / advice you could give. Thanks to all who reply.

John
Mesa / AZ

[heep70]

Lift the truck and spin the tire count the revolutions the yoke makes. I will guess 3.73.

[Faithful Old Road Dog]

You'll want to get higher gears (lower numerically). You'll need to spin those tires a bit slow because the other tire will want to rotate the opposite direction. That's if you have an open rear diff. You do know to spin your tire one full rotation while counting the rotations of the yoke, right?

[heep70]

If it is open, leave one tire on the ground or divide by two.

[sandman]

Ok, I am very familier with working on vehicles but please help me here.

I have never jacked a vehicle and spun the tires to find this out. Please explain if you can on how to do this ?

Thanks.

John

[heep70]

Jack the rear up spin the rear tire one full turn and count how many times the driveshaft yoke spins around in a complete circle.

If while turning the rear tire and the other rear tire spins the other direction you have an open diff. You need to secure that tire so only the one in front of you spins.

If the other tire spins the same direction then you have a locked rear diff and you can continue to count the revolutions the rear yoke turns and not worry about securing the othe tire.

Did that confuse ya, i mean help ya?

[sandman]

Ok, just one last question.

When I count the turns of the yoke, including the partial ( 1/2 or 3/4 turn ) is this then the gearing of the rear end ?

IE... does 3 full complete revolutions of the yoke mean that I have a 3.0 gear ratio ? or 3 & 3/4 revolutions of the yoke mean I have a 3.73 rear end.

Thanks again.

[bigdoug22]

if the gears havnt ever been changed then the gear ratio would be on your plate on the door but yes assuming that if open rear and you kept the non drive wheel from turning your gear ratio would be as you described. your truck sounds like mine i thought i had a camper special it isnt it a custom same engine and tranny i have 3.73 dana 60

[jor]

IE... does 3 full complete revolutions of the yoke mean that I have a 3.0 gear ratio ? or 3 & 3/4 revolutions of the yoke mean I have a 3.73 rear end.

Yip. That's exactly right. There are lots of variations of this tire/driveshaft scheme. The way I do it is to wrap a long plastic tie around the driveshaft with the tail of the tie sticking straight out. Then turn the driveshaft so the tail of the tie is pointing either straight up at 0 degrees or straight down at 180 degrees. Then mark the tire you have jacked up and mark that same place on the pavement beneath the tire. Now, slowly turn the tire and have someone else count the revolutions of the driveshaft. As you noted, 3 and a half revolutions would indicated you have a 3.50 rear end ratio. Good luck and post how you do.
jor

[fordman]

yea check your door plate.

[sandman]

Thanks guys. I am pretty busy with work so I will try to get to it by this weekend to see what gearing I have.

I dont have a door plate on the truck or I would have checked this first.

I bought this truck about a month and a half ago and it has a new VIN number assigned to it by the state of Arizona. The old sticker is there on the door but it is blank, like someone wiped a chalkboard with an eraser.

I was getting a few parts from the junk yard here last week and saw many other trucks with that also had VIN numbers assigned to them the same as mine.

Thanks to all who posted.

[Jake11]

If you take the cover off, it's stamped on the ring gear.