Changing the spark plugs won't help, unless your current ones are fouled. In that case, just replace them with new ones specified in the owner's manual.
Bigger alloys and wheels increase fuel consumption do result in higher fuel consumption. The rotational mass is increased, so the engine has to work harder to spin the wheels. Acceleration and handling is also affected. Change the alloys and wheels to the stock size when the wheels are finished.
Also, try to keep the revs low - shift at 2k rpm, and avoid sudden or hard acceleration.
