Robert,
Of course, your fuel mileage will vary depending on your driving habits (speed and acceleration rates), coach weight, wind resistance, and terrain (hills,mountains, or head winds), but the entire line of C 12 engines in Beaver coaches from the 425 HP to the 505 HP version will give better performance and fuel mileage than any other big block engine. I have been driving a C 12 powered Beaver for the last 15 years, a 425 HP 2000 Marquis a 505HP 2002 Solitaire, and I have averaged from 8 to 8.5 on the 39,000 lb. Marquis to 7.5 to 8 on the 50,000 lb. Solitaire that is 13.5 feet tall. Another comment about the C 12 is that in about 200,000 miles of driving one, I have found the engines to be almost bullet proof.
Gerald