Larger wheels and mpg?

Unless you're going to an ultra-light type rim, you'll increase rotational mass when you increase your wheel size, which will always effect your mpg. The effect on mpg depends on the increase. I went from stock 16's to 18's on my 03 V6 coupe, and it decreased my mileage by 1.5-2mpg. You may be experiencing the 'fun-to-drive' effect from the change. The increase in footprint will improve handling, and you may be getting on the accelerator a little more to feel the most out of the improvement. This will impact your MPG more than the wheel change.

Unless you used a tuner to recalibrate your speedometer & odometer my guess is that you're now running a taller tire 7 wheel combo, which rotates less over a given distance. Put another way your odometer is registering slightly less than the actual distance driven ( as is your speedometer). I ran into this years ago when putting bigger tires on a pickup truck.

Marine One said:

Unless you used a tuner to recalibrate your speedometer & odometer my guess is that you're now running a taller tire 7 wheel combo, which rotates less over a given distance. Put another way your odometer is registering slightly less than the actual distance driven ( as is your speedometer). I ran into this years ago when putting bigger tires on a pickup truck.

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+1
I'd say that is your biggest reason in MPG reduction. A tuner allowing you to the change the tire revs per mile will fix you right up. I went from the factory 17s to the factory 18s and haven't really noticed any drop in MPG