I'm going with weight and diameter.
Diameter will alter the calculation for MPG. OP really hasn't said much about the new tires/wheels so I can't offer much in the way of how much error.
Thought I'd made a thread discussing tire diameter errors, but I can't find it.
Rotating weight is always in play, even at sustained speeds. It takes more power to keep it at speed.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.