What you want for best mileage is for your engine to be at peak torque. An engine (no accessories), running at full throttle (we're ignoring power valves and other enrichment gadgets) has its lowest fuel consumption/unit power produced at the torque peak. The horsepower peak is a mathematical artifact of increasing RPM coupled with decreasing efficiency, and is not usually a good place to run for max. economy.
OK, now a little quick theory: The reason that economy (efficiency) maximizes at the torque peak is because this is the place where the engine is inhaling the greatest amount of fuel and air into the cylinder which, when burned, makes the peak amount of cylinder pressure and thus the maximum torque.
So for the tire part, they are heavier and therefore harder to turn, ultimately sapping power, as if there was a small load on your alternator. Also, with the tires at different diameter, you would no longer for example have the engine at the RPM's where its generating peak torque at your typical highway speed i.e. 55 mph. Tieing this together, thats a manufacturers goal, get your cars engine at peak torque development while driving at your legal speed limit while in your top gear
