The over all reading on mpg from the computer is false. One rotation of the larger tire makes you travle a longer distance the the oe tires. so your wheels turn less to travel the same distance. So your probably driving faster. Your computer thinks you still have stock wheels so even your mph are off. Seen it thrown off by up to 10 mph befor. Wind resistance is a parabolic curve in relation to speed. so faster you drive more it pushes back its like your speed squared. I get 28.5 at 65mph but 23.5 at 75 mph. Also the wider the tire the worse the milage. The 18s are probably an inch or more wider which will reduce mpg. And if you went chrome they are heavy which also will reduce mpg. All in all if I were to guess it would be you are driving faster because you are getting a false reading of your speed from your computer and its killing your mpg.

 

That would depend upon the manufacturing process of the wheels being compared. I replaced my 16" factory alloy wheels with 17" aftermarket wheels that are chrome plated and shaved almost 40 lbs. of unsprung weight off the vehicle.

 

My chrome rims weigh like 30 lbs each. but your right core metal aluminum instead of steel would make a lighter rim.