2017 T-680 Cummings 10speed auto

I've got a 2017 t680 with an isx15 with a 10 speed auto and get right at about 7 mpg hauling a normally heavy 48' flatbed. It's also a midroof geared at 2.64. Hope that helps at all.

 

Make sure tire pressures are up. Clean air filters.

Cummins used to be notorious for failed turbos. Don't know about the newer trucks.

My last truck was a 2012 with 18 speed. It had some power. I averaged around 6.8 in the summer pulling 100k one way. Local gig. The weekenders would get about 5.5. No idea how they could burn more fuel and oil on 2 days then I do on 5 days.

Saw that truck rolling down the highway last week. Blowing black smoke galore. Can't even see the taillights on the trailer anymore unless the brake lights are on.

If you're running the western region. You're probably getting about right with the mountains. You need mid region flat land do get decent economy.

A good mechanic can test the turbo to make sure it's spinning full speed. Using laptop software. If it's not spinning. It'll choke the motor to a degree that economy suffers. You'll see black smoke.

 

I have the same transmission in my 2018 T680 and I think it is the worst transmission in any vehicle I've ever driven. The guy who works on my truck said that it has a notoriously bad reputation and he thinks it's just a big hunk of junk.