Downspeeding confusion

I just read an article saying that basically a truck setup for down speeding should only be used on relatively flat land out on the open highway. Fleets having troubles are in regional settings or in the mountains.
If the truck is spec'd to run 1150 at 60 or 65, then it likes to run at that speed or higher. If you run it too much in an area where you can't stay in top gear, then the constant torque on the driveline at low rpms causes u-joints to fail prematurely.

 

Look up "gear fast- run slow".. it was common.. but if you give some of these apes a tractor that will run 100 mph.... it would be foolish..

 

When one spec's out a truck, they have to account for where they will be running, the roads they will have to run, and the weights they will be running, among other factors. I typically will spec something that will allow me to run at normal Interstate type of stuff in the top gear at the RPM that is ideal for the engine, then also be able to run slower two lanes a gear lower and keep it in the same RPM band... Always keeping the RPM's at the optimum balance of economy and power. Most of the fleet stuff spec'd via advice from OEM's and Dealers is around the idea the someone will be operating only on flat ground with no wind. I am all for lower RPM's when it can be done prudently. But some of these fleet and OEM dweebs have some stupid mindset that low RPM's are the primary concern to get fuel economy and don't understand how overcoming rotational inertia of the motor and being at the low end of max torque can be more of a detriment than a positive. If one operates a motor outside of where it actually has the most efficient balance of economy and power, they are usually not improving their fuel economy and probably hurting it. And that is usually not down low in the RPM range, close to the bottom of the max torque curve. And in doing this improperly, driveline stresses are increased and subject to forces that are unnecessary, leading to more frequent problems.

 

No. The engine would hate running down there. Newest engine we have is a 2008 so I can't speak for any of the newer stuff.

 

I typically cruise at around 1425. Detroit 60 12.7. A fiend of mine has a 2015 Prostar with a Cummins ISX and he has it set up to cruise at around the same RPM, averaging around 8 mpg. The down speeding idea has some merits, but only in a limited application. It is hardly a generic one size fits all idea that will work for everyone. It is the current idea du jour and is being promoted by OEM's and being sucked up by fleets in that never ending quest to reduce fuel costs, but the results are only there is specific applications.

Even Mack/Volvo understands this more than the other OEM's when it comes to their transmission setups. Their latest auto shift options have one that will only have the engine operating at those low RPM ranges being pushed nowadays when the truck is empty or on flat ground and lightly loaded. When loaded to gross and working non flat terrain, it goes into a different mode that keeps engine RPM's up in that 1300-1400 range for maximum power balanced with fuel economy.

 

I thought 1400 was down speeding.

I can see using that setup if you have a dedicated route that has you deadheading eastbound across the plains for 50% of your miles but other than that............

 

I pull a lot of bulk commodity loads on rural two lanes in some hilly areas. I have found 1425-1475 is just fine in doing that with the 12.7. I rarely touch 1500 or go over it except on shifts or if I want to "goose it" and get around someone.

 

Say what? A few short years ago he was pushing the idea onto his flock. Hmm imagine that.