1999 Dodge Ram 2500 Turbo Diesel

I just bought my first Dodge Diesel so far I like it. I was wondering about tires. I have had Discovery AT on my 1990 Suburban and I was wondering about the Discovery ATR vs. the Stock Mitchlen also what kind of fuel mileage should I expect running empty cargo on the interstate. It is a heavy duty version should I increase tire size from the LT245 R75 /16? Thanks Bill