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I think I can answer the question of what tire size is best. I've read that rolling resistance is inversely proportional to tire diameter. This means that a larger diameter tire will require less power and will improve gas mileage, assuming that engine RPM stays the same. So, it is probably best to install the largest diameter tires that will fit the car. The aerodynamic drag may go up because larger tires makes the car higher, so the fuel economy benefit may be reduced at high speeds. This could be adjusted by changing the springs. The reduction in engine RPM with larger tires is also likely to improve fuel economy on the highway. It is probably best to increase the rim size instead of the tire height for best handling. I'm not sure if there is a best tire aspect ratio but some low drag tires have an aspect ratio of 65%.
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