differential gear ratio for the 4.6 swaps?

[RICH MUISE]

What are you guys that have done/are doing the 4.6 with the 4R70W tranny swap using for rear end ratios? I'm going to have to address that sometime this year, and I think what I remember being recommended is a 3:50 (3:55?) gear ratio because of the OD. I don't want it to be a dog "off the line", but my primary concern is reasonably low rpm's at a 70ish cruising speed.
I do want to keep the oem '57 case, but I'll have it narrowed and set up for discs.

[Ford Blue blood]

My 10 F150 is 3:55 and runs 2250 at 80 MPH.  The Mustang is 3:73 and runs 2500 at 80.  The 93 F150 with a 3:55 ran right at 2300 at 80.  The 66 Falcon Ranchero with a Mustang 5 speed and a 3:23 ran...no idea...at 80 but made gas running 65 - 70, it sounded like  it was in the mid to high teens.

[57AGIN]

Rich:

Don't forget the trucks tires are much taller than what you will be using so the engine RPM would be lower. My transmission guy tells me your 4R70W's overdrive ratio is 0.70.  That's a good starting point for this discussion.  My Shelby GT with it's 3:55 rear gears gets as high as 24 mpg when cruising long distances at 75 - 80 mph.  The final drive ratio in my overdrive is 0.71 and my RPM's @ 80 mph are about 2300.  My tires are probably a bit smaller diameter than what you will be running.  My guess is that either a 3:50 or 3:70 gear would give good all around drivability.  What the heck, if you don't pick the right gear to start with, your Ford 9" has the ability to take pretty much any gear you could want.  Enjoy your final assembly and road testing, you have one heck of a great ride.

Bob
57 AGIN

[lalessi1]

The 4R70W has a 2.40 first gear, that seems very tall. If f you go with a 3.89 rear end ratio that gives you the equivalent of a 2.72 overall ratio on the road. running 26" diameter tires you should see 2110 rpm at 60mph, 2813 rpm at 80 mph. That seems reasonable. A 4.10 would be better for acceleration. this would give you 2224/60 and 2964/80. Personal preference. These rpm/speed/tire diameter numbers are directly proportional, in other words a 27" tire would reduce rpm by 26/27. A 3.55 ratio would reduce the rpm by 3.55/3.89.

[RICH MUISE]

Good info, Bill
Thanks Bob...I think you just unknowingly(or knowingly) cleared up some confusion I had..3:50's or 3:55's. From your post I'm assuming what your saying is the one available for the '57 9" are 3:50's, and it's the newer differentials (8.8's?), like your Shelby, and Bill's trucks, that have the 3:55's. Is that correct?
No, mine's a budget build, and I'm guessing around 1K-1200 just for the tracloc gear set, so If I don't get it right, I'm gonna be stuck with it no matter how many other options might be available.
Lynn, I'll have to digest all that info.

[lalessi1]

Wasn't trying to confuse the issue....

Rear end ratios depend on the number of teeth on the pinion.  A 9 tooth pinion can have ratios of 35/9 or 3.89, 34/9, 32/9 or 3.55. The numbers have to be whole since you can't have a fraction of a tooth on a gear. A 10 tooth pinion will give even different numbers... 35/10 or 3.50. Math fun with cars!!! 

[RICH MUISE]

Well I may finally be seeing the light on the differential mystery...So, the 35,34,32 numbers come from the number of teeth on the ring gear, (if that terminology is correct).. Do the '57 era differentials have both the 9 tooth or 10 tooth pinions available?

[lalessi1]

I am pretty sure just looking at the ratios that '57s all had 9 teeth pinions. Just a guess, but I think the pinion number of teeth stays constant for a given differential design.

[canadian_ranchero]

the gear ratios for the 4r70w are 2.84 first,1.55 second,1to1 3rd,0.7 od.i am using a 3.56 axle ratio[ford used 3.56 till 1969 then went to 3.50].this gives me a 10to1 overall first gear ratio.and with my tire size will be turning about 2100 at 70 mph in od   

[lalessi1]

I saw 2.4 on the web for late 80s early 90s Lincolns. Thought that was very tall.... 2.84 makes more sense. I think you must look at the overall gear ratios to pick a rear end ratio.

[Ford Blue blood]

The 57 Ranchero has a 3:56 (tag is on the housing), the 57 Ranchwagon had a 3:89 and the Edsel has a 2:91.  All are 9".

The 8.8s are 3:55 in the truck, the GT500 had 3:55s and I put 3:73s.

The Explorer rears (95 - 01) came with 3:55, 3:73 or 4:11.

No idea how the pinion tooth to ring gear thing works, I just count both and do the math.....:<)

Rich if it were me I would use the stock 3:56 as my 98 MK VIII LSC was a 3:55 and gave great milage as well as performance.

[lalessi1]

9 teeth and 9 inch no relationship, I agree.... do the math. If one has a 3.50 or a 4.10 ratio they don't have a 9 tooth pinion....

[RICH MUISE]

I'm pretty sure my oem is 3.56....I took the tag off years ago to clean it up so I could read it, and lost it. If I was keeping the single drive axle, I'd probably just keep the stock gears, assuming they checked out ok, but I am wanting to change to the trac-loc.
As long as I'm getting an education here...where/what does the term "tall" come in, or, what does it mean?

[canadian_ranchero]

if your car was a 6 cyl. std trans,they usually came with 3.70 axle ratio.tall,long legged,high speeded is used to talk about a ratio closer to 1to1   

[57AGIN]

Rich:

If I'm correct, and of course I think I am (lol) I believe the following to be the way it is.  If you have a "tall" gear in the rear end it is a gear that is meant for very good gas mileage.  While a "low" gear ratio is one that is suited for quicker acceleration.  When I was young this just didn't make sense, as we all know that a 4:11 gear was the one used back then if you wanted acceleration  (now days a number of quarter mile racers use a 5:38 ratio and buzz their motors to 10,000 rpm).  The thing that didn't make sense was a "low" gear had a larger number than a "tall" gear.  That's bass ackwards, isn't it.  But, that is the way it is, at least I think it is?

When I had my 65 Mustang I had two gear sets, a 3:50 and a 4:57.  I can tell you quite positively that driving my 65 between LA and San Diego to and from home to my ship board duty station that if I had gone racing that weekend with the 4:57's installed, I was turning about 4,000 rpm at normal freeway speeds.  The rpm's dropped considerably when I ran the 3:50's, but so did that wonderful feeling of getting pushed back in the seat when the foot hit the gas pedal to get onto the freeway.  Man those were really good old days.  The same hold true today, before I put in the TKO 600 with it's 0.64 overdrive gear, when I wanted to change lanes on the freeway I didn't have to down shift into 3rd.  All I had to do was hit the gas pedal and I'd accelerate like a bat out of xxxx and be in the next lane.  Now I have to think about making a leisurely lane change in 5th gear or dropping down a gear and stepping on the pedal.

With the overdrive in the 4RW70 you plan on installing having a slightly lower rear end gear shouldn't cause you problems.  It will help getting you car moving and once you get into the OD gear the rpm will come down to not much over 2,000 to 2,300 rpm. 

Bob
57 AGIN