Ok here is my 2 cents. I have a 98 Cummins. The book calls it a CTA 8.3 ltr 325 hp. The fuel delivery system is all mechanical for the most part. The coach tips the scales at about 32000 lbs. I also tow a 3000lb car on a tow dolly. I am averaging about 9 - 10 mpg. I have been across this great country 7 times. I am not the first to the top of the hill but I am also not the last. Going down hill I just down shift the Allison, apply the Pac Brake and cruise safely to the bottom. It is my belief that the coaches are coming with larger engines not so much because of their weight but to make up for the loss of HP that the pollution control******that is mandated. JMHO :)