15 or 16 inch rims?

Bigger tires burn more gas.

 

To be precise, bigger wheels -- usually heavier -- and slightly wider tires
burn more gas.

Technically, this is correct. Theoretical MPG reduction may be as much
as 4% or maybe 2 MPG. Keeping the tires filled to 42psi front/40psi rear
goes a long way to overcome this. On either size wheel, the choice of new
tire can result in a loss of up to 6 MPGs -- there are usually other pluses
such as improved handling, less noise, better braking, etc.

In everyday driving the combination of variables such as temperature,
traffic, topography, route, tire pressure variations, and even driver attention
level make it virtually impossible to "isolate" this small fraction of overall
MPG/FE.

A more significant difference is there are considerably fewer tires on the
market to choose from for the OEM 16" tire size.

There are many threads discussing the relative merits of the two wheel/tire
sizes and numerous tires. The search tool is your friend!