The increased diameter of the rim + the increased rolling resistance due to wider tires needed for bigger rims will bring down your mileage more than weight unless the difference in weight is huge...like you putting dubs on your hybrid. Duk has 17' wheels on his hybrid and even though they are lighter than the stock hybrid wheels his mileage decreased. I switched mine to 16' CSX rims and my mileage dropped approx 3mpg but thats with tires that came stock on the CSX. You may want to consider low rolling resistance tires.
Heres a thread with stock wheel weights
OEM Wheel Weight Revealed! but I dunno if they have the hybrid wheel weight on there you'll have to search the forums for that one. You will need 3mm spacers on the front to clear the brake caliper, rear has drum brakes so you may be fine without spacers.