The mileage numbers you provided are misleading. While they represent a range, many factors influence fuel consumption, making the only valid mileage calculations based on a substantial number of miles.
Factors that can skew mileage calculations include speeds driven, headwinds, hills, weight, the levelness of the fuel island, when the pump shuts off, idling time, generator usage, fuel composition, outside temperature, and elevation.
That said, if you calculate the total fuel used over a distance of at least 5,000 miles and do this consistently every 5,000 miles, the true mileage will not vary by more than a tenth of a mile from segment to segment. I have a logbook documenting about 500,000 miles of coach driving over 34 years. My 1997 12.7L engine initially averaged 8.0 MPG, but after the introduction of low-sulfur fuel and other changes, it dropped to 7.5. My driving habits are consistent; I drive at 62 MPH (1,500 RPM) and only deviate from that speed when I’m not on the interstate. I also tackle hills aggressively.
My 14.0L engine started at 5.9 MPG, but after the EGR function stopped working, it has consistently achieved 6.4 to 6.5 MPG at the same speeds.
Mileage is unaffected by available horsepower but is significantly impacted by horsepower used. For example, pushing against wind at 50 MPH requires less fuel than at 70 MPH, even if your steady speed on flat ground remains the same. Using more horsepower to climb hills faster will also consume more fuel.
Before making any changes, gather accurate data by calculating mileage over a considerable distance and recording your hill-climbing speeds during elevation changes. This way, you will have a solid basis for comparison to measure any true changes if you modify your coach.
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