2017 Force 37BH HD....I averaged right around 7 mpg on my last trip. 2,300 miles, 6 family members and a dog, towing ~8K, from Denver to OR coast and back...so a lot of hilly terrain at altitude. I was running at a pretty good tick most of the time, 70'ish.
From Cummins (
https://cumminsengines.com/uploads/d...el_economy.pdf)
Rock-Solid Rules:
1. Above 55 mph, each 1 mph increase in vehicle speed decreases mileage by 0.1 mpg.
2. Every 10 psi of underinflation represents approximately 1% penalty in fuel economy.
3. Every 2% reduction in aerodynamic drag results in approximately 1% improvement in fuel economy.
4. Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.
5. Used lug drive tires can get up to 0.4 mpg better than new lug tires.
6. Ribbed tires on the drive axles provide 2–4% better fuel economy than lugged tires.
7. The break-in period for tires is between 35,000 and 50,000 miles.
8. Tires make biggest difference in mpg below around 50 mph; aerodynamics is the most important factor over around 50 mph.
9. The most efficient drivers get about 30% better fuel economy than the least efficient drivers.
10. Idle time is costly. Every hour of idle time in a long-haul operation can decrease fuel efficiency by 1%.