This will most definitely decrease gas mileage due to heavier tire and bigger diameter. Also that will affect performance, but not by much. Probably not even noticeable. But on the bright side - ride will be better, more meat
It will probably slightly increase actual gas mileage. It will reduce apparent mileage.
It will make your mileage warranty last longer.
It will increase the odds of getting a speeding ticket.
It will reduce acceleration.
It will interfere with putting chains on your tires.
Based on GEOLANDAR G055™ - tires installed on my RAV4
255 v 235
Diameter difference is 0.7 inches.. maybe will affect speedometer by 1 mph - no worries about speed difference.
Weight difference - 255s are 6 lbs heavier - will lower MPG for sure.. donno how much lower, maybe 1mpg, maybe more, but i wouldn`t say it will be something drastic. For reference, when i had Corolla, i installed 2 sizes up tires, MPG decreased by 1.5, roughly.
Width - 225s 1" wider. Good for dry surface.. maybe decrease of traction on wet (puddles) or snowy/icy roads.
But again, nothing major that should scare you, if you want to do it, do it. I know i will when tires are due for replacement. Will go 2 sizes up for sure.