I drove over the road 18 wheelers for about 15 years, and worked for several trucking companies. Most of them bought small horsepower (300 - 350) diesel trucks to pull up to 80,000 pounds of freight up and down the mountains out west. At best, they averaged 6 to 6 1/2 mpg. At an average of 100,000 miles a year, that is a lot of expensive diesel fuel. Some of the trucks were "streamlined" to get better fuel mileage. Generally didn't happen enough to measure. The last 5 years that I drove, I owned my own truck, and hauled the same type of freight and trailers that I drove for the companies. My truck was a Western Star, with a classic square blunt nose truck design. (non aerodynamic) It had a 500 horsepower diesel engine, with the same 9 speed transmission that the company trucks I had driven had in their trucks. With the extra 200 to 250 horsepower, my truck did not have to work as hard as the company trucks did to haul the same freight. My overall average fuel mileage was 7 to 7 1/2 miles to the gallon. Add that up over a 100,000 mile year, and it is a lot of money savings in fuel alone. Plus the added reduced wear and tear on the engine. Bottom line is that the extra power allowed the truck to do the job with less effort and reduced wear. The truck had over 1,000,000 miles on it when I sold it and retired. The guy that bought it drove it for another 4 years with the same engine in it, and without a major overhaul. YMMV.
Blessings,
Omar