Downspeeding confusion

This was 5-6 years ago. Didn't take long to find an old thread from his message board where he recommends low rpm...this one is from 2008. As for his flock....he used to have....(maybe still does I don't know)....a band of loyal disciples on his message board, the likes of which have not been seen since ancient Rome. I would guess he's off the bandwagon now that his orphaned signature truck failed

"Kevin and many callers seem to think that Cummins is out to lunch in recommending that their customers gear for no less than 1450 RPM at 65 MPH for Maximum Fuel Economy."

http://letstruck.com/forum/topics/whats-so-wrong-with-1450rpm?commentId=526987%3AComment%3A549754

 

Lets not forget that the engine first off needs to be capable of efficiently operating while downspeeding........as well as the proper spec of drivetrain(usually +1 size increase at least).

Problem is many fleets are pretty ignorant to this and have spec'd equipment that isn't compatible with the objective. Example would be downspeeding a motor past it's low end power capabilities and/or undersizing the driveline to "save a few $$"

Older motors are virtually impossible to correctly downspeed because their Tq curve falls off way higher in the RPM band anyway. The Volvo motors will hold FLAT Tq down to 1050 before it falls off. Most of the new ISX motors hold solid to about 1150 before fall off. A LARGE part of why this is possible is the VGT/VNT turbo technology that allows boost to hold dramatically lower in RPM than older motors with fixed geometry turbos.

Generally you want to cruise at least 100 RPM above the Tq fall off and expect one backshift on approach of a grade. On a Cummins ISX, you'll want no less than 1250 rpm, Volvo 1150, Detroit DD15 1200 rpm minimum cruise.
......And remember your driveline need to be up sized at least +1 to handle the Tq correctly.

 

2013 and newer ISX engine the torque will drop of at 1100 rpm. Expect it to be a lower RPM for future engines. I drove a truck with 3.08 rear end gears for about a year. It did just fine and I was able to do 8mpg with it at 78,000 lbs. The whole down speeding thing really does work. I was very skeptical about it but after seeing numbers on fuel mileage I was convinced. You just have to worry that when you go stupid low on the gearing then you have to upgrade your drive shaft and u-joints to handle the additional torque. It truly is a revolving door at which part will break when you change one little thing.

 

The fleets that like this idea for the improvement in fuel mileage, seldom keep trucks past 400-500K, so they don't see the long term torque hammer issues.

 

With the biggest complaint being U Joint life, just change them every 250,000 or whatever you think is best.
U Joints are pretty darn cheap.

 

http://www.volvotrucks.us/powertrain/application-specific-solutions

 

down speeding sounds like lugging to me

 

It can be. Depends on load, terrain, wind, etc. The one size fits all thing that OEM's and fleets do with spec'ing trucks can be more problematic than helpful. It takes knowing what you generally do, and running various gear ratios and transmission setups to determine what works best.