I purchased a 70 GT (351C-4spd) that's suppose to have a 3:25 open 9" rear. Before I parked car in storage, I jacked it up and verified it wasn't a posi. Checked the third member and unfortunately there's no tag. I'm assuming it's a 28 spline axle, but have read a couple articles that claim Ford put some 31 spline axles in the open rear ends? I'm not going to get back to pulling an axle on the car for awhile... so can anyone tell me if my hunch on the stock axle being 28 spline is correct? Why??? I'm shopping for a posi setup and thou a 31 spline is desirable, I've seen a 3:50-28 spline posi third member for sale at a pretty reasonable price. Also, I measure my 9" rear ends width and it's about 56" (backing plate to backing plate). Not being able to accurately measure while the wheels/tires are on... I'm not sure if it's total width (axle face to axle face) is 61" or 63" ? Both widths I've read were put in Ranchero's?
No real way to tell that I know of without pulling an axle and counting carefully, however, I suspect that you are correct that it is 28 spline. I seem to recall a marking or design dimple on the face of the axle that "suggested" one or the other but it escapes me at the moment - Perhaps Hillbilly or Carl (5.0 Chero) knows off the top of their heads... Traction-Loc differentials (Ford's version of posi...) are readily available. What are you looking to achieve from your car? Burn the tires off or... Bear in mind that 3:50 to 1 ratio is going to limit your top end speed, will make your RPM considerably higher at freeway cruise speeds, and will decrease your fuel mileage. My '72 351 4V 4 Speed with 3.25:1 Traction-Loc is about 3,000 RPM approaching the 65 to 70 MPH range. I had another car with 3:00:1 Traction-Loc that I was happier with for today's freeway speeds. I had 3.50:1 in my 429 Ranchero, fuel consumption high at 70 which sucked (9 MPG). Higher rear ratio = easy to burn the tires off (cool if that is what you want), lower rear ratio = more gear for the top end speed (and the M code 351 4V can still burn the tires off with 3.25's).
Not an absolute, most 70 and 71 rear axels conform but there are a few exceptions. Center of the rear axel you see while looking at it from the outsides, if it has a cast in depression sorta' rectangular shaped mostly is a 28 spline axel. If the center surface is flat with what looks to be two drilled shallow holes it probably is a 31 spline axel. Remember, the only sure answer is to pull the axel then count the splines. ID tag may or may not match what you have.
Thanks for the info guys. I typically install limited slip gear sets to "not burn the tires off" as much. I'm running a 3:50 set in my Fairlane with an AOD trany and it works well in our rural area. Understand the wide ratio 4spd with higher gears gets worst mileage, but I really didn't buy this toy for the MPG rating. Life is short, enjoy it while your here. I have an old license plate trim ring that's been on several of my trucks thru the years.." There's more to Life that Gas Mileage". Happy New Years!
Actually - the wide ratio 4 Speed is much better IMO - taller first gear (as in - no granny gear), and 4th gear is 1:1, same as any. With an AOD, yes, the 3.50 is a good choice, and closer matches what factory are doing with those. I asked about the ratio desired simply because I've found "cruising" and the driving experience so much nicer with a lower gear ratio - however, your car, do what you feel agrees with your driving styles.
You can tell if it's 31 spline by looking at the hub on the axle (part that centers the drum). The 28 spline will have a large single rectangular indention and the 31's will have a small hole in center and two larger holes on either side. Edit: Just saw Hillbilly's post on this.
