How do I figure out the gear ratio of a rear differential that has the tag removed? The one I'm looking at (Craigslist) is out of the car but is the complete axle end to end. The seller says he's sure it's a 31 spline and either 2.73 (I assume he means 2.75) or 3.00 gears. Thing is it's a bit of a drive to get it so I want to know what it is before I leave. Am I correct in thinking I can turn the differential one time around and that should give me 2.75 turns at the wheel hub? Is there anything written on the axle case (inside or out) that will identify the ratio?
Seller states it's out of a 75 F100 which from looking at the "FORDification" rear axle ID website leads me to believe it's probably a 3.00 geared unit not a 2.75.
This is actually going in my 69 Galaxie but I figure the information is useful and kinda relates to our trucks as well.

Thanks for any help.
Scott.

 

I believe it's the other way around. Turn the yoke and see how many turns it takes for a wheel to make one revolution. Take some chalk or tape with you to make witness marks.

 

So what's it from, a car or an F100?

9" = 1973/79 F100; 1975/79 F150 could have either 28 or 31 spline axle shafts.

1975 F100 9" could have 2.75; 3.00; 3.25; 3.50, 3.70 or 4.11 ratio.

If from an F100, you cannot use the entire rear axle in your '69 Galaxie. It's too wide and the Galaxie has coil spring rear suspension.

You can swap the center section, but first...you need to verify the spline count.

btw: 1969 Galaxie could have the WER 8.7" integral rear axle, a 9" or a 9" with a 9 3/8" ring gear.

 

Got it thanks John.

 

Yeah I'm after the center section only. Unfortunately my 69 galaxie has the oddball 9 3/8" rear differential, it's a decent unit but good luck getting a truetrac or anything else aftermarket for it. Thanks for the information ND.

Scott.

 

If the 3rd member is out of the housing, all you have to do is count the number of ring gear teeth, the number of driving pinion teeth, divide the ring gear tooth count by the pinion gear tooth count and you'll have exactly what the ratio is.

Good chance the car 3rd member will have a cast-in 'hood' sticking out off the driving pinion housing. It's for the pinion snubber that would be on a car. It won't be useful on a truck but it won't cause any problems running it on a truck. Another thing is the cars tend to have longer companion flanges than trucks (the flange sticking out off the stem of the driving pinion where the driveshaft attaches). If this is the case with this passenger car 9-inch 3rd member, it may cause problems with the existing driveshaft length.

 

Look at the end of the axle shaft, the part sticking through the brake drum or rotor. It's been my experience that if the end of the axle has a depression, or is hollow, there is a pretty good chance the axle is a 28 spline. If the end of the axle is flat and smooth, it is usually a 31 spline.

This isn't a foolproof test, but only a generalization based on my experiences.

Good luck.

 

An oval depression on the center of the axle flange register was common on the 8-inch rear ends in the early Mustangs and related Fords.

Two divots on the center register isn't a definite determining factor between a 28 and a 31-spline Ford axle.

My '69 F100 came with 28-spline axles in the 9-inch rear end. I pirated a pair 31-spline axles from a '71 F100 that had a 9-3/8" rear end (Ford 9-3/8" rear end is a close cousin of the 9-inch but not the same as).

Stock 9-inch 28-spline axle shown in the rear end housing in my '69 F100, in photo below. '71 F100 9-3/8" 31-spline axle shown beside it. Other than the difference in casting numbers on the center registers, the only main notable difference between the two is that the 28-spline axle has two circular rings cast into the register. The face of the 31-spline axle register is smooth, other than the divots.



28-spline axle being removed to be replaced by the 31-spline axle --after I swapped the 3rd member out for a 31-spline Traction-Lok differential.



31-spline axle installed.



The stock '69 C7AW-E 9-inch 3rd member on the left (28-spline, single track, 3.50 geared) was replaced by the Ford Daytona D0OZ-B 9-inch N-case 3rd member on the right from a '75 F150 4x4 (31-spline, 4-pinion, Traction-Lok differential, 3.50 geared).

 

Thanks everyone for the replies I've written the various ID methods down, It'll take a while but pretty sure I can find what I need now.

P.S. I'm jealous of that nodular rear diff you have there Ultraranger.

Thanks.
Scott.

 

I have two factory 9-inch N-cases. The one I put in my truck is the Daytona version --notice the pinion housing doesn't have any depressions in the casting between the 5 bolt heads.



The other N-case I have is not the Daytona version. It has the regular pinion housing where the casting is raised up where the bolt heads are (not the same thickness all the way around like the Daytona pinion housing).