They are the quickest of the bunch, no doubt about that one. My dad is the bus mechanic for the school district here and they have a newer bus that has a cummins in it and the big weak point on it is the electrical system. The gm sales person stopped there one day and said that to get a new bus with a duramax in it is $10,000 more than the vortec 454. You will never make up that difference over the life of that bus no matter how far it is driven.