Ok, guys here's your chance to help me out AGAIN. I finally disassembled the 9 inch rear I bought many years ago when I was 1st acquiring parts for my project. Everything looks pretty good, will replace bearings, races and seals, and check tooth contact (3.00:1 open diff).

While I have it apart, this would be THE ideal time to change R&P out. I'm thinking 3.50:1 might be the ideal compromise here.

BTW my drivetrain will be a fairly mild 390 with a C6. Anyone running either ratio and have any comments? Appreciate your feedback as always

Ron
http://rons55ford.blogspot.com

 

Ron - Our setups are similar. I am running a 300 hp 351C/FMX and P295R50/15's in the rear (27" tall). My truck came with a one-legged 3.00:1 9-inch and I changed it out to a 3.50:1 posi.
The 3.00 was a perfect road gear (70 mph at about 2500 rpm) and I got 15-18 mpg.
The 3.50 gets it off the stop light and is a lot of fun around town, but winds the heck of it at freeway speed (70 mph = about 3200 rpm). Of course, my gas mileage sucks (9-12 mpg).
If I was running an AOD, the 3.50 would be a perfect world with good grunt off the line and freeway puttin. If I don't get an AOD, I'm thinking about taller tires, which will hook up better anyway.
With your setup, my vote would be to stay with a 3.00. If you hot up that 390 at all, I really like the posi. It's a little squirrely in the rain, but that's just part of the fun.
My two cents.

 

Like buying a computer, you first need to define what you want to do with it. Stop light drag racing, trips to the strip, short <100 mi trips with full burble of exhaust, leisurely weekend cruising, daily driver, heavy stop and go traffic, long distance traveling with minimal gas consumption, heavy towing/hauling. etc? higher numerical ratios 4.xx -5.xx:1 will give stronger accelleration, more pulling hauling power, higher rpms, more noise, more gas consumption. Ratios in the 2.xx to low 3.xx
will give high gas milage low engine/drive train noise, less shifting in traffic but in a heavy truck will accelerate like grandma going to church. Mid to high 3.xx are mid range universal ratios. Slant towards one end or the other to find the comprimise that works best for your personal preference. Tire diameter will also need to be considered.
There are a number of sites on the internet that have RPM = speed calculators where you can plug in the rear end ratio and tire diameter and calculate the effects on speed and RPM. You can put in the highest speed you're likely to want to drive at and a safe max RPM for your engine and see what ratio works, or you could use a lower speed and RPM for say a daily driver where you drive every day on local highways averaging 45 MPH max and prefer listening to Motzart for relaxation so you want the engine @ < 2K RPMs during the commute. If you want quick push ya back into the seat accelleration but need good gas milage for long distance cruising consider using something in the 3.9-4.1 range but add an aftermarket overdrive unit or upgrade to an OD tranny.
One good thing about the 9", you've got an almost unlimited choice of ratios available.
Our Panel has 3.70:1 rear a 400 SB "other" motor and a T56 6 speed manual double OD tranny. Good low end power, close ratio first 4 speeds for stoplite fun but interstate cruises @ 80 MPH shows less than 2K on the tach and 24 mpg at the pump.

 

With a big inch motor and an automatic there is really no need to change. That setup will even tow a camping trailer or boat with no problem.
If you really want a bit more pulling power than go to a 3.25 as the lockers for them are more common it seems, at least around here.

Now if you had a girly motor 302 then Id definitely head for a 3.50 or even a higher ratio. But a 390 is not what Id consider a high winder.

As Ive mentioned several times on the forum Ive had to drop to a 2.75 with 460's just to get a bite. All a 3.50 does is create smoke.

 

This trucks hauling days are pretty much behind it, except for a couple folding chairs heading to the car shows. I would like a little more grunt that the taller gears would give me, but good highway manners would be nice too. It's a tough call, but I'll probably leave it as the 3.00 for now. I guess one of the many pluses of the 9" is the ease of gear swapping if/when I change my mine. Of course, if I find a relatively cheap 3.50 posi 3rd member at a swap meet, who knows? Another bonus I found out was that I have C7AW-G casting with the "big" carrier bearings.


Ron

 

I agree with 286 Merc. That 390 has plenty of oats to push your truck around town with a 3.00 gear. The sensation of it revving quicker with a 3.50 gear may seem cool... but in reality the acceleration of your truck over a quarter mile will change little, if any. In fact, more tire smoke means slower E.T.'s.

I had a '67 with a 390, auto and 3.00 gear and I could stomp it off from a dead start and the thing just went... much to the chargin' of some kids in a '72 Chevy pickup with a big lumpy cam and flowmasters one day... just about the time they caught up with me... I let off.

Sam.