Deeper OD, 5600 or 47RE?

It seems I turn just a few RPM's less than my dad's 6 spd Cummins with my auto Ram.

Which tranny has a steeper ratio in OD?

 

The 6spd is .74 to 1 and the Auto is .69 to 1 in O/D so thats why you see a difference.

 

Interesting.....

So does the 6 spd version actually get any better mileage than the auto or is that a myth with today's hi tech lockup converters and such?

 

You won't see any better fuel mileage with a Dually reguardless of what tranny you have compaired to a single wheel due to the rolling resistance.
Everything I've read states the manual will give a better return with fuel mileage because of the drivetrain coupling loss through the torque converter before converter lockup. The clutch will always be a 1:1 through all gears in the manual trans.

 

like said before torque converter slippage

 

Vs. rolling resistance of the Dually. Might just cansel each other out.

 

I'll bet the Dually is worse due to aerodynamics and weight compaired to a SRW and more so a short box.