BAD HTM SR 24,
Congratulations! If you like the truck now, just wait until you tow with it. I have an 01 2500hd 4X4. I have been averaging 15.8 in the city. Your mileage will start out at 12.6 and continue to get better. It takes around 5,000 miles for the motor to loosen up.
I hate the 245's. There are some issues about going to bigger tires with the Allison. The dealer can reprogram the speedo but the shift points for the Allison get messed up. I have seen a few Dmax's driving around with 285's. They look sweet.
Go to one of the diesel sites and do a search on tire size and Dmax. You should be able to find some information out there.