Class 6 is a Medium Duty truck. Ford offers a 200hp Cummins or a 362hp gas engine in their F650. Have no doubt which one will tow faster up the hill. Cummins offers the 6.7L for small motorhomes. The 8.3L, ISL9 and ISX15 are offered for the big guys. On our next cruise, I'll ask if the 6.7L is pushing our ship through the Caribbean. The Cummins B series does make a cool novelty engine for the family car though.:B