larger tires have less rolling resistance. you'll use a little more gas around town, but out on the highway on a long trip you'll gain a little mpg. my father in law always brags about a 97 merc. sable with a 3.0 v6 got 36 mpg on the highway. with 4 adults and chucked full of luggage on a cross country trip.

 

A 285 of the same tire design compared to a 265 is not going to have less rolling resistance. It will be slightly heavier and require more energy to get them moving but not enough to think about. I think the 285 look good on a 2500 but I would not do it if the 265 tires were still good. I have 265s on my truck mostly because they needed new tires as soon as I bought the truck so I spent no time researching tire options.

 

yes, it has more mass. it's like push mowing your lawn with a push mower with small wheels, and then push mow your lawn with a mower with the taller wheels, you'll notice it's easier to push the mower with the bigger wheels. i'm not talking about the weight of the tire. but a taller tire will roll more easily than a smaller diameter tire. especially over rougher roads, or bumps.