Back a few years ago a concept called progressive shifting came out that was to aid truckers in saving fuel and wear on engines. The concept is based on gradually raising the shift point as you progress through the gears using the torque value of the engine not horsepower to pull the load. Cruising RPM's were re-adjusted by changing gear ratios to allow running at highway speeds at peak torque not peak RPM. The Cummins B is an industrial truck engine so it does not hurt it to run at peak RPM all day (2600-2800) and under some loads on inclines this will be necessary. However the fuel milage could be increased by reducing RPM's while maintaining highway speed ie; 3.54 gears-5th gear OD-2000 RPM-65-70 mph. The concept was to backshift to 4th(direct 1-1 ratio) when pulling an incline= peak torque pulling (1400-2000 RPM) at cruising speed versus (2500-2800) peak horsepower at approximately the same speed 60-70 mph. You will see posts on here about lugging your engine (def: any time acceleration is not possible at a specific RPM and speed in any gear). Now you will realize on certain hills that no matter what gear/rpm/speed/load there is no way you will be accelerating but simply trying to maintain any speed. I would recommend on the highway basically staying above 1800 RPM, in town running around empty lots of times my RPM will not exceed 1800. With a mechanical engine the possibility of hurting your engine exists by over fueling at low RPM's ( holding the pedal to the floor creating black smoke) otherwise the engine doesn't really care. As well the turbo will be more efficient at lower RPM's but high enough to maintain close to maximum boost (18-25 psi stock) for optimum fuel milage pulling. This is going to sound confusing at first but after a while you will feel the engine and drive accordingly. Too much low RPM/speed driving (city only) will cause excess carbon build up and will hurt the engine so just do a highway cruise about once a week and let the engine work and clean itself out. PK

 

I drive up to 50-55 mph with the OD off when in traffic. This means she tachs around 2000 rpm with the 3.54 gears. I also run about 15 miles of my daily commute each way at 70 mph - also 2000 rpm in OD. My mileage stays in the 19.5 - 20.5 mpg range even though I do stop-and-go everyday for a few miles each way.

I do not like the way these auto trans try to go into OD at 45 mph because the torque of the motor will really launch you forward at the most inopportune times, it seems. By driving with the OD off with it locked up in 3rd and letting the motor rev you do not hurt anything and you get some engine braking when you have to back off the pedal.

As PK said, letting it rev does the motor more good than harm. You also get to hear that wonderful growl out of the pipes ;D

Your mileage may vary, offer void where prohibited, use at your own risk.........