Burnin rubber

[Fordf100++]

Hello everyone. Just looking for some advice on a possible rear end swap for my 1970 f100. The truck was my Dads and the id I found on it is C2AW4668. From what I found it is from a 1962 galaxie and I believe my dad had it in there for cruising. Can anyone give me advice on what rear end or gear ratio I can go to so I can burn some rubber? I am running a 390 with a c6 tranny. Thanks

[ultraranger]

3.50:1 is generally a good all around ratio but at highway speed with a C-6, you're going to be around the 3,000 RPM range with the engine.

There were various ratios and differentials produced but, the three most common ratios in the Bumpside F100 trucks were 3.25, 3.50 and 3.70:1.

The axle ratio codes can be found on the tag affixed to the trailing edge of the drivers side door.

The axle code for a 3.25 ratio is 17.

The axle code for a 3.50 ratio is 08.

The axle code for a 3.70 ratio is 09.

The 9-inch differential axle gears and the axles in the Bumpside trucks will be 28-spline. Dentside trucks will have the same codes but, the majority of the '73-'79 F100/F150 Dentside 9-inch rears had 31-spline axles/differentials.

The above codes will pertain to open (single track) differentials. This means you'll be making 'peg leg' burnouts with them.

There were 28-spline Equa-Lok 9-inch differentials in the Bumpside F100 trucks '67-'68 and Traction-Lok differentials in the '69-'72 models. The axle code on the door tags will have an alpha-numeric (instead of two digits) on them --A2 for example.

Ford Equa-Lok differentials are not good for high horse power or for high torque. The Ford Traction-Lok differential was much improved over the Equa-Lok but, it's best to stay away from the '69 model Traction-Lok due to (4) large holes in the left half of differential casting --They are prone to breakage in this area.

In 1968 ('68-'72), Ford also produced a 9-3/8" rear end for the F100s. These rear ends came with 31-spline axles. If the 9-3/8" rear end came with a Traction-Lok differential, the 31-spline axles would have axle bearings with a 1-5/8" I.D. x 3-9/64" O.D. If the 9-3/8" rear end had an open differential, the axles would have axle bearings with a 1-17/32" I.D. x 3-9/64" O.D.

I have a pair of 31-spline axles that came from a '71 F100 that had an open differential 9-3/8" rear end. I have several 9-inch 3rd members with 31-spline Traction-Lok differentials, that I had pulled from various Dentside trucks.

I swapped out my stock '69 F100 C7AW-E single vertical rib 9-inch 3rd member,with a 28-spline open differential with 3.50 gears, for a '75 F150 9-inch N-case 3rd member with a 31-spline Traction-Lok differential and 3.50 gears.

My stock 28-spline single track 3.50 ratio 9-inch 3rd member on the left. 31-spline Traction-Lok 3.50 ratio 9-inch N-case 3rd member on the right.



My stock '69 F100 9-inch 28-spline axle on the left. '71 F100 31-spline 9-3/8" axle on the right.



31-spline N-case 9-inch 3rd member installed in my '69 F100 9-inch rear end housing.



31-spline axle from the '71 F100 9-3/8" rear end installed in my '69 F100 9-inch rear end housing.

[tnlprt]

Final gear ratio also depends on the size of the rear tires

Short tires can run 3.25 gears and have the same effective rate as having tall tires and running 3.70 gears

3.25 gears for cruising

3.50 gears for being in the middle of the pack

3.70 or 3.89 gears for improved out of the hole performance

Like I said Tire size makes a difference also in the final ratio

The Ford 9in is the poor mans quick change rearend.....With a little practice and 2 guys with air tools you should be able to change gears in less than 45 minutes with another 3rd member that is setup and ready to go

Here is a gear and tire chart for reference.. There are plenty more on the web

http://www.jeepsunlimited.com/forums/sh ... res-amp-bs

[Fordf100++]

Thanks for all the info. Tire size is 275 55R17. With this in mind, anybody have any specific rearend(year and ratio) I can find? I would like to keep the rearend I have but find another pumpkin I can swap out for burn outs. Thanks