Just did 1,423 miles empty from McCammon ID (south of Pocatello) via I-15 south, I-84 east, I-80 east to Rochelle, IL (just south of Rockford, IL). Left McCammon's Flying J with full tanks and cruised at 1,300 RPM and 61 MPH to the Pilot truck stop here. The tanks took 166.5 US gallons to 'full' which means my fuel mileage over the past 4 days has been ... 8.7 MPG!!! So, whoever says CAT engines are fuel guzzlers, take notice. What I think helped me achieve this MPG:

1. new air filter installed 2 weeks ago
2. air-to-air hose clamps have been tightened, and one extra clamp has been added; after this pulling power increased and boost went up
3. extrude honed exhaust manifold
4. performance freeflow muffler
5. tires: pressure has been checked after the cold weather set in
6. fuel: anti-gel in tanks
7. engine speed: CAT recommends to keep the RPM between 1,300 and 1,350; the trick is to have a DIGITAL tachometer because the dashboard one will always lie (mine says "1,250 RPM" when the digital says "1,300").
8. trailer: my Wilson step only weighs 10,100 lbs and iis very low (minimum air resistance).

maybe you had a tailwind

Half of the time it was blowing sideways, and there were quite a few hills.

Wyoming hwy 80 has some fierce cross winds.
Does this include idling time?

I just did almost the same trip...lots of west wind blowing this week. Did Portland to Chicago 3 weeks ago, same route, 8.2 mpg with 10,000 in the box. 550 hp, 63-67 mph, and no fancy gadgets.

My point? One trip doesn't mean a thing, it's all about conditions. Did the same trip last week and didn't even make 7 mpg.

Most trucks will do better than drivers think for fuel mileage when driven right. My '06 International 9400i with ISX, 13spd, 3.42 rears averaging 35,000 lb in the box consistently busts 7 mpg year round. That is all fuel for all miles and idling and calculated (not the dash display). Not uncommon for mid to high 7's during warmer months and low 7's for colder months. In real bad weather with high winds and heavy loads, I rarely get below 6.5 mpg on a bad day. When all the stars are aligned and the winds are right, even loaded I can put away 8+ mpg.

Good numbers, Tracer!

I dunno man. You shoulda looked at your map Tracer. You coulda turned right out of the J and cut a good 200 miles off that trip by coming across US30 to LaGrange WY and Little America on I-80. Fewer hills, less traffic...and better scenery!!

How fast do you usually drive? At what RPM?

No, no, no. I actually did a delivery in Soda Springs, Idaho down that Highway 30. Going 2 times over that monumentous mountain was enough. Google Maps says - McCammon, Idaho to Little America, WY: 185 miles, 3 hr 25 min via Hwy 30; and 245 miles, 3 hrs 54 min via I-15, I-84, I-80. So, the route I took was 60 miles longer not 200 but I lost only 30 minutes which means Hwy 30 route is slower, if you divide miles by driving time. 54.25 MPH average for Route 30; 62.8 MPH average for I-15.

I don't idle, I use my bunk heater when it's cold. Unless it's 0F (-18C). Then I might idle for an hour before going to bed - if the truck sat for a day or two. When my battery is properly charged, the truck starts okay.