13 Speed Fuel Mileage

You need to know your engine power range. Also run your engine near to above torque always. All the info you need is on the engine badge.

 

Yeah, it's rated at 485 with 3.25 gears.

 

On my engine the horsepower is noted at 2000 rpm's. I rarely get it up that high.

 

Never say never. I am not an engineer. It has a lot to do with terrain and load. Todays fleet truck has RAR rear axle ratios of well below 3:55 a ratio I would not advise as your gradeability will suffer tremendously, but if you are running on the flat and your weights are way under gross, say 30K then this gearing could produce the 10 mpg salesman will use to sell you a truck. I am using a 2011 and when my mileage drops below 6 I know the DPF needs regen or the venturi in the aftercooler needs decarboning ( see Rawz ). It is mostly as the exhaust is so restricted it negatively affects power and mileage. I am told that after 2014 the exhaust restrictions have been solved so I think you have some problem with your system if you are getting so low a mpg. I think the engineers have described the sweet spot as being another 200 spins a minute lower than you are using, at least that is what i was told by a cummins service writer, but I agree with you 1400 is a better place to be for power availability. I believe the best mileage will be in a direct gear 1:1 instead of the overdrive usually found in todays top gear and certainly w/ a 13 hole. But the terrain/ load is probably the biggest determinant. It is hard to make such broad statements as I did and have them speak truthfully to every situation. But in my operation, coast to coast, north of rte 80 w/in 10% of gross I will stand by my statement.

 

Double over is TWO overdive gears generally a 1:0.83 and 1:0.79. I think ( and who really cares what I think) that a dble over in todays world would be a VERY specialized operation. Downhill for very heavy. Cheyenne to Grand Isle with maxi fuel , return MT.

 

This graph shows the key about fuel economy and speed. Increasing speed increases rolling resistance linearly, but aerodynamic drag increases exponentially.

The full article is here: https://www.nap.edu/read/13288/chapter/7#83

Cummins has a good publication as well:

https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf