T680 questions.

[JimT]

(02-15-2020 )Mattman Wrote:  My door sticker says 17,672 for weight.
Not exact as the truck you are looking at but should be close.

I have 3.36 gears and 295/75R22.5 tires. Wish I had 3.55 gears I have to be going fast to use top gear while loaded. Mine is 450hp 1650 torque.

I feel you. I have 3.42 rears with a 13 speed. 63 mph in 13th puts the engine at 1350rpm. 1450rpm puts me at 68. If I drop down to direct (12th) then 1550rpm puts me at 62mph. 1800 in direct puts me about 70mph. 1350rpm gives me the highest mpg but is only doable when light and on relatively flat ground, high 7's to low 8's. 1450rpm (at 68 mph) still gives me 7's for fuel mileage even when loaded around 70k. 1550 in direct (62 mph) isn't bad but averages about .5-.75 mpg less then 1450.

Of course, all those numbers are with cruise control set and NOT climbing any significant hills. Prior to finding this site I would keep my operating range 1300-1500, rarely ever hitting 1600rpm. If I let it drop down to about 1250 I could feel it lugging, learned pretty quick on my own to stay above that. (Yes, I know, I also cringe now at the thought) NOW I KNOW BETTER and keep normal operation between 1400-1600 with grades and acceleration hitting 1700-1800 when appropriate. Only time I run below 1450 is with the boost under 10psi and at 1350 I keep the boost to 5psi max. If that can't hold I downshift.

Still getting the hang of it and finding the sweet spots, but without the dpf active my fuel mileage is good. Once the DPF starts to regen though, I lose AT LEAST 1 mpg off my trip average. Despite all the work I've done to get this system operating properly, I think it's time to cut my loses. I estimate the cost of continued maintenance along with the extra fuel usage will cost me about $15k per year to keep some air clean...