Technical 1958 Ford 9" Third Member

Thinking of lowering my axle ratio, to bring my revs down a little on the highway. I have a 1958 Custom 500, with the 223 six & a Ford-o-Matic....Granted, the "ideal" would be to back it up with an AOD, but I don't have that kinda cash right now, so thinking of slipping in either a 3.00 or a 2.91.....I know it's gonna knock a little off my acceleration, but I used to drive my '60 Falcon w/ the 144 & a Mercedes 240-D, so I'm kinda used to driving a slug.....question being: What sort of interchangeability am I looking at here? Seems like almost everyone I talk to has a different story.....Ideally, I'd just like to get the whole third member assembly & just bolt it in, because I don't have the skill & patience to get involved with the lash/shims/preload, etc, when you just change the gears......Thanks!

 

Ford 9"s interchange easily. You really only need to check three things; axle spline count (28 on yours, and the most common type), make sure you have a fill point (if it's in your axle housing, no worries), and the driveshaft yoke. This last item may be the sticking point as Ford used multiple types with different u-joint sizes and lengths, make sure that matches and no driveshaft mods needed. You could swap yokes if needed, but some skill is required to re-set the pinion bearing pre-load when doing this.

As far as ratio, a 3.0 would be the highest I'd try. I think you'd be happier overall with a 3.25.

 

See if there is a tag on the third member. Good chance you already have that ratio.

 

Do you have any idea what ratio is in it now? My 57 Bird with an automatic came stock with a 3.15:1. A lot of big Fords with automatics in the 60's and early 70's had 3.00:1. I pulled a complete rear axle out of a 64 T-Bird that was a 3.00:1. That's what I have in my Model A. It's a bit "high" (low numbers = high ratios) for what most of my buddies recommended, but it's got a 401 Buick and it doesn't have any trouble with that ratio.

 

My BIL claimed his 58 [FE engine] had 2.49 rear gear. I didn't believe him till I read it somewhere. I'd bet with the 223 it's probably close to 4.11, I got one from a 62 6cyl station wagon with 3.89 and it's in a 59 housing in my OT Comet. Follow Crazy Steve's advice but I would check see what you have now.

 

Mark a tire and the drive shaft push the car one full tire rotation and have someone count the drive shaft turns.

 

The tag on the front door frame says axle code 2....is that the 3.56? If so, would going down to a 3.00 even be worth the time & expense? How many revs would it really cut things by, at 60-65?

 

A 3.25 would reduce RPM by about 10%, a 3.00 about 19%. Even 10% is a fairly big jump, with the amount of power you have an almost 20% jump may find you in 2nd gear much more often which would kind of negate the whole idea.

 

28 spline 3.00 and 2.75 are pretty common. About the only thing that doesn't have them is the F150's which are 31 spline and usually 3.50 in the 70's- early 80's. Vans have 28 spline. Try and get the same length of pinion yoke, there are two different lengths an inch different. A bigger or smaller U Joint is easy to adapt but driveshaft length isn't.

 

If you have a 26 inch tall tire and go to a 30 that is a 15% gear change If they will fit a 235/80/15 is 30"

 

My '59 came with a 3.56 rear, I now have a 3.89 in it. You can run a RPM scenario through a rear end gear calculator. Your 1st gear is a 1.75 to 1.

I copied and pasted this from a Hemmings article.

But for 1959, Ford introduced an essentially all-new smaller, lighter, simpler and cheaper two-speed Fordomatic, undoubtedly in advance of the 1960 Falcon and Comet. Note that the new two-speed Fordomatic was the only automatic available with the six and the base 292 V8 on the ’59 full-size cars. Also note that Chrysler’s two-speed Powerflite was also available on the large cars through 1961 (but not with the six starting in 1960).

The new Fordomatic had an aluminum case, and a simpler two-speed planetary gearset. Low (starting) gear had a 1.75:1 ratio, and the torque converter had a maximum stall ratio of 2.6, meaning that the maximum effective starting gear ratio was 4.55:1. That was almost as good as the Powerglide’s 4.73:1 max gear ratio at start. Note that both of these are well below the typical manual first gear ratios of the times.

 

Get a better sound system in the car, so you don't hear the engine so loud....it's quite happy driving 65 mph with 3.56 rear gears. It was designed that way.

The yoke is the biggest thing about the swap that could cause problems. They had a short, small yoke on those rears. But the 9" is the easiest one there is to deal with swapping the yoke. You can take the pinion support out and play with it separate from the rest of the third member.

But really, try driving it for a few years with the current set up, see if you can get used to it. The car doesn't mind.

 

Should you need to change over the pinion yoke it's easy to remove the pinion assembly from the center section and replace/preload the crush sleeve.

 

The FOM is close to a Powerglide as far as 1st gear goes and lots of the old Chevys had either at least a 3.23 rear gear. So it shouldn't perform out of the hole too badly.