[sciencegeek]

I was repeatedly warned by the dealer not to alter the tires as a small percentage difference in rotational distance can mess with the computers.

From the way they describe it, the car is constantly measuring and adjusting the motors from front to rear and left to right and if you start messing with the size of the wheel you can cause problems.

Common dealer "buy only our wheels" tactic / positioning. Has same experience with BMW.

Exactly. If the tires are all the same size (they better be) but different from stock there is no issue except the inferred distance traveled.

If you get tires with a larger diameter, your actual distance traveled will be greater than what the car 'thinks'. Therefore your consumption will show higher. If you get tires with a smaller diameter, it's the opposite. That's it. Nothing else.