Before you buy newer; the mpg isn't there.

I'm getting 6 mpg with my stock '13 with a isx 600. That's at the pump, after running almost a thousand miles. I do short runs, 60 miles, 120, 51.. About 500 miles/night
I don't idle, run it like the book says. I try to keep it right at 1400 RPM/68 mph with my 18 speed and 336 gears, but sometimes people get in my way, sooo......
Want to tweak it some later. Maybe a Pittsburgh power appointment. I'd love a 0.5 mpg increase!

 

At that mileage your IMAP, Delta P and EGR Back Pressure sensors should all have been changed out. The IMAP is especially notorious for killing fuel mileage when they get clogged.

 

there are so many ways to economize of fuel usage easing back on the throttle is the best way there is a very good reason why Swift have their trucks governed at 60 mph the other wastage is when parked up for the night lots of trucks leave there motors running to cool/heat in one night that adds up to as much as 8 gallons of fuel x 250 days = 2000 gal = @ $ 2.00 per gal =$4000 x amount of trucks in fleet.

 

My dad's isx 475 is getting between 7 and 8 hauling open deck and his truck has a 120.inch sleeper. Lots of idle time with what he does loading and unloading. We put a turbo blanket from Amazon and a Donaldson m1000463 straight thru muffler on which improved it between .3-.4 tenths. He runs the lower 48, runs about 65 mph, 18 speed and 3.55s. Still need to trick the fuel pressure sensor with a inline resistor that supposed to increase torque, hp and fuel mileage, and we took care of/ eliminated the emission issues.

 

My best truck was a 16 FL dd15 @515 hp. Pulling flatbed. 6.7 in the rocky mountains.

My next best was a 96 T600 D60 @400 hp with that crappy super 10 speed. Pulling flat. 6.5 in the mountains.

Everything else was in the 5.5 to 5.8 range. One 05 w900, One 13 FL, the rest were 09 pete's and kw's.

 

I get 4.3. And I run 68 or less.

 

What year is the 660? 3.55 is a fast rear end for newer trucks, combine that with faster highway speeds and shorter miles I'd say that MPG isn't that far off. I run a 13' mack MP8 and average 6.5 - 7.1 depending on the terrain (1400 RPM, 65mph). The RPM's are so important tho keep in the sweet zone for MPG.

 

I get 6.4 running 72 mph loaded (77,000) and a tad bit faster than that empty. 495 miles per day... exactly half are deadhead.

Idling the main engine any significant amount knocks it back to 5.5. It gets 5.9 on Monday if I stay out and run the APU over the weekend.

2007 Cat NXS in a Freightliner Cambodia, 10-speed, 3.42 gears, Goodyear G399LHS all the way around, 1.3 million miles.

I'm not in any hurry to buy a new one.

 

Not an owner operator, but I poked my head in here at the fuel mileage info. I put 60k miles on a 2015 Kenworth T680 10-speed automatic with Paccar engine and Thermoking APU in my last six months with Stevens, as OTR, with a good bit of time in the Western and Eastern mountains. Governed at 62.

Average fuel economy over 60k miles was 8.6 MPG.