rear axle ratio to choose

Planning ahead for my 56 C series pickup build on my 2001 Dakota chassis. My goal is to use a 2008 or newer remanufactured 4.7HO with either a 545RFE or 65RFE trans. The rear axle in my donor Dakota is a 3:55 ratio with drum brakes. I'd like to plan an axle upgrade with disc brakes, but also work the option of rear axle ratio for overall driveability and decent fuel mileage. My reference point:

Back in 2006 my company vehicle was a Ford Explorer 2wd 4.6V8 (300hp) with a six speed trans and 3.55 rear ratio. I did a lot of highway driving, still do, and that Explorer surprised me with better than 22mpg on the highway at 70 mph. At times, loafing along at about 1600 rpm with a tailwind it would exceed 24mpg. So my 56 C series, will be a new 4.7HO V8 and 545RFE, would be over 310hp (headers, dual exhaust, intake, all the usual stuff), similar trans ratios and rear axle ratio, could possibly produce decent highway mileage above 20mpg. My goal is a fun daily driver that gives me grins when I get on it and grins when I rud steady highway speeds. I also want it to be a reasonable driver for my wife, who is not a gear head. So for an axle upgrade to get a strong rear axle with disc brakes, should I consider a slightly lower rear axle ratio to get my highway revs in 5th gear into the 1800 rpm range. I do plan to use the truck as a daily driver, haul my motorcycles, tow my 5x10 tilt bed trailer when needed. Some more perspective too. My current Dakota with the 245hp 4.7 and 5spd manual gives me adequate acceleration. So anything better from that perspective, with 310hp and much more torque (than the old Dakota), in a rear drive pickup would be a fun upgrade to me. If I stay with a 3:55 ratio I'll still get around 2000 rpm on the highway at 70mph. But a slightly lower ratio may still give me decent performance/acceleration and better gas mileage.

I think some Dakotas came with a 3:21 rear axle ratio. But would that be a decent rear axle for over 310hp/350 ft-lb of torque.

Hi Andy , Those 56 Trucks have Huge wheel wells . What size Tires are you planning on running in the Rear ? Makes a big difference in what Ratio to recommend to use . Ronnie

Very good point. My goal is to blend my 2001 Dakota (2wd) and the 56 C series into a 56 C-Dakota. So the rear tire sizes will be typical for a Dakota on the stock 16" factory mags, possibly on 17" steel smoothies. Either way basically the widest tire I can mount on a 8" rim width, as I plan to slightly drop the ride height and tuck the tires into the fenders. This will be a daily driver that I'll actually use for some hauling, carrying motorcycles, towing my 5x10 trailer sometimes. So something like a 255/70-16 on the factory mags, or a 255/65-17 on the steelies.

Andy, the rear fenders on your 56 (actually from 53 on) are the same rear fenders used on the Little Red Express trucks. The fenders themselves are 10" wide I believe. Your Dakota rear end might be narrow for the 56 bed, you may have to mini tub the bed sides for inside tire clearance. That makes the fender even wider.

If you plan on towing with this rig I would not go below the 3.55.

I don't know which way you are leaning more, toward daily driver v/s towing. If you aren't doing much of one or the other, you could always have two sets of different size wheels and tires to run on hand. Net effect being you are switching between 3.21 and 3.55 gear essentially.

an example using the .68 od of an A518.
3.91=2.65 and 4.10=2.78
3.55=2.41, and personally, i would feel this to be too high a gear for cruise rpm to deliver good gas mileage unless driven on relatively flat ground.
i don't know what the od ratio of the transmission wanting to be used is, but one can figure out what the final drive will be, easily.
remember though, going too low an rpm at cruise can negatively affect gas mileage if that rpm is out of the engine's efficiency range.
aero drag and weight will also play into this equation.

2.14 is real close to the rear gear ratio the M bodies, with the 7 1/4 axle, had stock. You aren't going to be saving any gas with that. I got better mileage when I swapping in the 2.94SG 8 1/4 I got out of a cop M body.

my point exactly.

Majority of my driving would be non-towing. I'd tow my 5x10 tilt bed single axle trailer maybe 500 miles a year if that much. East central Wisconsin is mostly flat to moderate grades, the kind where the cruise control in my Dakota doesn't sound labored at all to maintain speed on grades (with the 5spd manual trans and only 245hp). I'll sometimes haul something like a mid-size motorcycle or snowmobile. But most bed loads will be less than 500lbs, when I even haul anything in the bed.

But, clear this up for me. A 2013 Ram 1500 with the 4.7 V8 offers the 3.21 or 3.55 rear axle ratio. The 2013 Ram will certainly weigh more than the 56 C-Dak when done. With at least 310hp and a 545RFE trans how can the 3.21 rear axle ratio not be a reasonable choice for every day driving? At steady speed of say 70mph, the revs would be about 2300 with the 3:55 axle ratio (like it is now in my 2001 Dakota), with a 3.21 axle ratio the revs would be about 2,080. I know from numerous times driving my Dakota if I keep the revs close 2,000 (about 60mph) the gas mileage does increase into the 20+ mpg range (on a 2001 Dakota with 200,000 miles on it, 5-spd manual, 3:55 ratio). So I'm thinking ahead to see if I can get close to 2000 rpm at 70mph.

Andy, my 48 Plymouth coupe has a 1990 Dakota chassis with the mighty 3.9 V6 (listed as 170 HP), 5 speed (AX-15 OD), with a 3:55 limited slip, throttle body EFI, (all factory stuff, the car is probably in the 3,000 lbs range). I can tell you without a doubt that when I am pulling a few hills in western WI at 70 mph in 5th gear, the car can loose 10 mph by the top of some of those hills, with the throttle wide open getting a run at them. That car averages 21-23 mpg on the highway (I don't drive to get super mpg). I can tell you without a doubt the factory didn't care if the vehicle could maintain a given speed with any motor, transmission, or rear gear ratio, all they cared about was getting it out the door and lasting until it got past the warranty limits. A motor that is laboring hard, at low rpms, and getting poor gas mileage seldom needs costly warranty repairs, which is good for the car company. That 2013 1500 Ram with a 4.7 and a 3:21 gear probably won't need drive train repairs until after the warranty expires. The factory doesn't care if the truck is a slug to drive once someone took it home the first time.

Well then, 3:55 is probably the way to go.

I had 3.55s in my Ram 4x4 with the 4.7 V8. For what I used the truck for it did alright but, I will tell you my combination was slower than early 2000s 4.3L V6 GMC Sierra 2wd work truck. I believe if not all Dakota V8s came with the 9 1/4 differential. You could go with a 4.56 gear but, I think at 70 mph your rpm will be 2500 rpm at minimum depending on tire and wheel size.