I am looking at buying a chevy silverado truck and had a question about gas mileage. How much better gas mileage does the duramax 2500hd (heavy duty 3/4 ton) truck have over the duramax 3500 (1 ton) truck. Thanks!

 

they should have the EPA mpg posted at the dealership, these will be most likely both V8 diesel engines , unless you get the 454 ci gas engine on the 2500 model.
you're talking about two trucks with massive hauling/towing power used more for pulling rv/horse/landscaping trailer with top of the line features and above the $ 50k range so why worry about a few pennies more on fuel ?
i 'd recomend you get the truck that you need to do the job and make sure is a 4x4 it'll come in handy when backing up with a trailer or in severe weather conditions.

 

For what it's worth you'll be somewhere in the neighbourhood of 15mpg city and 18mpg highway.