Alloy wheels = More MPG?

Stock Prius alloy wheels are very light. Almost any other wheel is heavier. Fitting larger rim will make it even heavier since rim gets bigger and amount of air (that’s weights almost nothing) gets smaller. And bigger rims are normally wider too.
Weight of the wheel matters a lot more for fuel economy than other weight since it’s unsprung and rotating mass.

 

valde3 hit the basics. As far as rims go, lighter is more fuel efficient. However what will have a larger impact on fuel economy is the tires. Properly inflated low rolling resistance tires can significantly reduce fuel economy. However low rolling resistance tires often have tradeoffs - such as worse snow handling.

 

snow handling isn't a factor in tampa, but yes, lighter wheels and LRR tires inflated to 40psi will give you better mpg. there are several choices for very light 16" wheels. bigger than 16" not too many. I have ecopias, and they give good mpg, but handling isn't all that.