I know you're being facetious, but...

There's a limit to how small you can go and still save on gas. The smaller your wheel diameter (rim + tire), the faster the wheel has to spin to get you to the same linear speed (i.e. at any given road speed of X mph, a wheel that's [e.g] 18" in overall diameter [rim+tire] will spin 10% faster than a wheel that's 20" in overall diameter). If the wheel spins faster, you get increased frictional heating in the bearings, and the grease won't protect as much, leading to increased friction, etc. At some point, the extra torque required to overcome the extra friction will essentially negate the "savings" in torque due to the smaller wheel. (Not to mention that if you spin TOO fast, your bearings will give out.) There is an optimum wheel size that will properly balance the spin friction with the wheel's moment of intertia to minimize the required torque and hence maximize gas mileage. If one has faith in the designer of the car, one would think that the stock wheel size (stock rim + stock tire) should be near optimal. Thus it is likely better to go with lighter rims/tires that are the same size as stock (e.g. by using alloy rather than steel, or with more intelligent spoke design), rather than simply going to a smaller diameter.