18” wheels gas mpg.

What are the weight of the wheels? What specific tires?

There's several factors. The wheels may be heavier, and the weight is further out from the centre, so more centrifugal effort involved, to get them rolling. The tires have a wider contact patch, and again are heavier, and their weight (centrifugal again) with low profile and 18" diameter is out near the tread face, compared to a 15" 65 series.

I'd guess the mpg drop is normal, to be expected. I'm living with the 17" OEM porker rims (something like 24 pounds apiece?), coupled with 215/45R17, even that takes a toll.

 

Once, on my 2007 Dodge Caliber, I found four sweet 17" alloy wheels with mounted wider & taller tires to replace my 15" steel wheels with smaller diameter Dunlop tires. I lost 1 to 2 MPG, & I was a careful driver. I had a CVT transmission that was tuned to get the best MPG at 2000 rpms AND also kept rpms as near 2000 rpm as driving conditions dictated. Therefore, any advantage of the bigger diameter tires for theoretical better mpg due to lower rpms, probably was lost. Later, on two other Hyundai Elantra cars(6 speed automatic & 6 speed manual), I switched to wider & larger wheels with mounted wider & larger diameter(+6% & +4%) tires. With those type transmissions, careful calculations showed no loss of MPG. At times, under specific conditions, I feel I gain a trace of MPG.