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Mileage numbers as given are wrong. They represent a range but there are so many factors included in why mileage varies the only valid mileage is based on a high number of miles.
Factors that screw up calculations on mileage range from speeds driven, headwinds, hills, weight, how level the fuel island is, when the pump shuts off, idling time, generator usage, fuel composition, outside temperature and elevation.
Having said that if the total fuel used over a mileage of at least 5000 miles is calculated and it is done every 5000 miles the true mileage will not vary from mileage segment to mileage segment by more than a tenth of a mile. I have a log book with about 500,000 miles of coach driving over 34 years. My 1997 12.7L over 8 years started at 8.0 MPG initially, and by the time they screwed with low sulfur fuel and other fuel changes it went down to 7.5. My driving is consistent. I drive at 62 MPH (1500 RPM) and only vary from that when not on an interstate. I do drive hills like I stole the bus, very aggressively.
My 14.0L started out at 5.9 MPG, and after the EGR function ceased working it has been consistently 6.4 to 6.5. Same speeds.
Mileage is unaffected by available HP, but is seriously impacted by HP used. For example if you push X amount of wind at 50 MPH, you are going to push a whole lot more at 70 MPH and use a serious amount of additional fuel. Even if you do not increase your steady rate flat ground speed, more HP used to climb hills faster than with less HP is going to eat fuel. Before you make any changes however you need to know facts so calculate mileage over a great deal of miles, record hill climbing speeds on elevation changes before making changes so you have a solid basis for comparison. At least that way you can measure the true changes if your coach is modified.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL