The door tag on my '66 Slick points to a 3.25 rear. Upon disassembly I discovered what appears to be a 2.50. Pinion is 18tooth, ring gear is 45. Obviously someone replaced the gearing but in further inspection the ring gear is only 8.5" across. I thought it should be 9"?

Any thoughts on what exactly I'm looking at here?
How does a 8.5" 2.50 gearing stack up against a 9" 3.25?



Link to all parts

 

John, I don't think there was a 2.50 rear but it surly it is not common.

Read this page when you get a few minutes, may help confuse you more. LOL Kevinstang's Ford Nine Inch Differential Page +




John

 

D7AZ-4209-B .. 9" Ring & Pinion Kit / 2.50-1 / 45 ring gear teeth / 18 pinion teeth.

Applications: 1977 LTD II/Ranchero/Thunderbird/Cougar / 1977/78 LTD/Mercury Grand Marquis/Lincoln Town Car/Mark V.

9" w/an 8 3/4" ring gear commonly found in 1965/74 F100's w/the 240 I-6 // 9" w/an 9 3/8" ring gear and 31 spline axleshafts was optional in 1968/72 F100's.

During the first Arab Oil Embargo (1973/74), most peeps became concerned about MPG for the first time. A second embargo occurred in 1979.

There were long gas lines at filling stations, some ran oughtta fuel. After the first embargo ended, a gallon of regular jumped from 25-30 cents to a buck.

Seems cheap, but it wasn't cheap, cuz a buck was worth much more back then. Some peeps said they would have to cut back on their driving.

So Ford began offering "High Speed" gear ratios in an effort to improve MPG. They did some other nutso things too, like offering 'light duty' Dana 44 REAR axles in 1977/79 F100/150's with 2.71 or 3.07 ratios.

And, if you think 2.50-1 is rare, there was also a 9" 2.47-1 ratio available in 1978/79 Passenger Cars. But, most 1965/79 9" cars came with either a 2.75-1 or 3.00-1 ratio.

btw: Beginning mid-year 1969, the 9's axle (differential) bearings changed from ball bearings to "slim line" tapered roller bearings.