This is why you have got to love the Pupster, come on Puppy stir some Poo Poo up!. The mower is only good at that speed if you can use it other than that some other z's may be better for most. Where I am most seem to feel the old XR7 is a little better at speed but those machines were underpowered. Pupster you tried to say with the math in your head the 72 would only be 12.5 percent faster than your 66 and your time was around 25% faster on the 72 than on the old 66. I found the same thing with my underpowered Super's as well the bigger deck just trashes the smaller ones in time on big areas. There is less slow down and turn time because you are not doing it as much + the deck is more efficient because you miss a inch or two on both sides while cutting and that makes the 72 more efficient as well so all this adds up to where a bigger cut just mows what is beyond your working it out on paper calculations tell you. I read on this site where a guy had a 60 DC then bought a 72 and said it cut a football field down form 45 minutes to 30 and that is 33% faster. I was just claiming 30% and people said I was full of POO POO. No doubt the more powerful engine keeps the speed up better but you pay for it in fuel a little. That's why I posted on here for most operaters from a 48-54 cut you go to a 72 and forget 60,but it is application dependent.