Are you running factory brakes? If so, they aren't designed for immediate stopping power(in case of an accident by anyone/thing).
No one here is insinuating you're racing. What he means by throwing off your speedo is by EASILY 5MPH or more. When I ran 17s(i'm dumped on the ground so it looks good...your 4x4 status will not), my speedo was over 7mph. I had to remove my fender liners b/c I was rubbing my tires raw.
Even if you're not lowered, 18s don't fit. Civic wheel wells aren't designed to support that large of a rim/tire combo unless custom fabrication is done.
Your gas mileage will go to s**t as well, and it'll be incredibly more difficult to turn your wheel.
Spend your money on repairing the car. With it being a 6th gen I promise you there are issues with it; it's an old car, plain and simple.