Most of us run between 4.5" and 3.75" bs on our wheels, regardless of the material they're made from. This allows for a wider than 10.5" tire.
Steel is heavier, as well as a larger tire will add considerable weight. Factor in added rolling resistance for a larger tire and it does increase fuel consumption. Personally, I lost about 1.25 mpg's when I went from my stock Canyons with 31's to soft 8's with 33's. Everyones experience will differ according to their tire choice, since some are much heavier than others, and tread depth also affects things. Wheel size will also affect the weight gain, and gearing and driving style/habits/terrain are variables to consider as well.