The rear axle in my donor 2001 Dakota for my 1956 pickup build is the 9-1/4 limited slip posi 3:55 ratio with drum brakes. Anyone ever run the 3:21 ratio? Probably more common in cars than pickups.

This past weekend we did a 600+ mile roundtrip up to Bayfield WI in our 2019 Kia Sorento, 3.3V6 (330HP) 8spd trans, 3:22 final drive ratio. For the trip the Sorento averaged over 27 mpg. Got me thinking, maybe a 3:21 rear axle ratio? I have to rebuild the diff anyway to replace the clutch pack, so I may replace the 3:55 ring/pinion set for the 3:21 if I can retain the limited slip. A 3:21 ratio with the 5.2 Magnum and 8HP-70 trans to give my even more daily driveability, I could sacrifice a bit of go at the pedal and just rely on the 8-spd to downshift when I really feel the need for more go fun.

I had a late 70s 9 1/4 that had a 3.21 ring & pinion, and it seems that gear was popular into at least the early 80s, but I don't know if that will attach to your newer stuff or not. As the emissions choked the motors, the manufacturers bumped the rear gear ratios to help the weaker motors.

If your building a rear anyway, it might be worth buying new R & P for of your gear choice. I think I would look to see if any rear gear companies offer the 3.21 gear for your axle. Be aware there may also be issues with the transmission speed sensors when different rear gears are present.

I'd say to drive it with the 3:55's for a little while before swapping out the gears. With that 8HP-70 you just might decide to go the other way (to 3:91's).
BTW, 3:21's are standard on some 2024 Rams, so you might find one in a junk yard.

Good points to consider. I've driven my donor Dakota since 2011, it has the 3:55 rear axle ratio and limited slip. The final ratio in 5th is 0.73 with the manual 5-speed, so I know what its like when I drive the Dak at 60mph or 70mph fuel use wise with the 4.7V8. When driving at 60mph in 5th, the engine revs are below 2,000 rpm and the gas mileage gets into the 20mpg range. My 56 will have a modded 5.2V8 in place of the 4.7V8. So if I can get the 56 engine reavs below 2,000 rpm at 70mph it will return better fuel economy (aside from aerodynamic difference between my 2001 Dak and the 56). The 8HP-70 final ratio is 0.667, so that will drop the revs at highway speeds. I defintiely want to keep the limited slip so I'll start by rebuilding the clutch pack in the axle when it goes under my 56. Like you said, paired with the 8HP-70 trans it may do what I want after I drive it a while. Then if I want, I can chenge to the 3:21 R/P set. I have not worked on a solid rear axle, but to change out a R/P set means taking off the diff cover and taking out the axle shaft clips to get the diff out right? Once the axle shafts are pulled out the diff carrier is the next step, so its in the realm of my wrenching experience.

Leave it in 4th gear on interstate, that will show what it’s like.

Excellent points. The 8HP-70 trans I got came out of a 2016 SRT Challenger with the hemi and a 3:06 rear axle ratio. So the 3:21 ratio will be closer. The 8HP-70 trans requires four inputs to operate; an RPM signal, torque range, TPS and a brake switch signal. The RPM signal can come from the speedo or the rear axle sensor (as it does on the Dakota). TPS from the throttle body. Brake light switch. And the torque is determined by Sound German Automotive in their controller package based on info I'd send them for the modded 5.2 Magnum.

I plan my 56 to be a year round daily driver, which means occasional lower traction surfaces. Since the 56 will be front end heavy, short wheelbase and over 300hp to the rear wheels I have to plan/design for that. I'll get a 22 gallon fuel tank in the frame behind the rear axle. I might locate some "ballast" boxes underneath the big fenders behind the rear tires. In combination that may get me about 400lbs on the rear axle. The real axle with have the limited slip, and I may use a distributor that I can run two ignition maps, one for normal driving and one slightly retarded to adjust lower power to the drive wheels. If I change the rear axle ratio to 3:21 that will also reduce the likely wheel spin on low traction surfaces. According to Russel at Sound German Automotive the 8HP'70 doesn't matter for rear ratio as long as the tire size and rear axle ratio are not at extremes.