In the rear you won't notice. In the front turn the wheel lock to lock and make sure they will not rub.
IMHO I doubt 18's on a 6.5hp will be noticeable unless the rubbing is really bad in the front and you pick a tire that is really heavy. Make sure you have the steering and fender clearance for the bigger size. When I went with 11x8-23s on my kinroad I had to adjust the fenders and they rub when I really crank on the steering.
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