Well, probably not ... think of a curve that plots engine rpm vs. road speed; when you went to larger tires, your rpm for a given speed actually went down. So in theory, you should have seen a slight uptick in MPG ... now this is usually offset by the increased rotational inertia of the larger tires, so often in practice, folks don't see any change in MPG (or it goes down, if they're not compensating for the larger tire diameter).
If you change the axle gears to a shorter ratio (larger numerical value), your engine rpm for a given road speed will go up ... so you'll likely see a reduction in mileage. If you change by the same percentage that you added in with the larger tires (need to look at the revs/mile specification to determine what that percentage was), you should be back to stock MPG and stock-like performance ... IIRC the math I did when I was looking into whether I wanted 4.10s or 4.56s for myself (I decided on 4.10s ... waiting on a couple of more big purchases, before they'll get installed), the 4.10s actually take you back past that 'stock-like' point to a point where the gearing will be slightly deeper than stock. So the performance you could expect will be slightly 'snappy-er' initial acceleration, slightly improved crawl speeds (lower), slightly reduced top-speed, and slightly worse MPGs.
