Yes, there is no law in United States. We can run any DOT tire we want, different rim sizes, etc.

Yes, we been saying for years that your speedometer will be off 5-15%

See Tires and Speedometer

If your goal is to save fuel economy, forget spending hundreds on new tires, and spend $200 on a 5-speed (the going price in WA). It will increase fuel economy more than tires will as it is about 85% speed reduction in fifth gear.