What brand are your 18s? Lesser quality aftermarket wheels are notorious for problems like this, hub-centric or not. So get the wheels independently checked out. If they're a lesser brand get some good ones, Tire Rack is a great place to start and ebay is a place to be avoided. 
After the wheels is tires: I would not exceed the proper size on the tire. I'd be shocked if you could go to that size and not rub on the front.
FWIW, here's what the tire numbers mean. And it's both in answer to your question and you'll shortly understand why.
The first number is the cross-sectional width at the tread expressed in MM, so a 245 is about ten inches wide.
The third number is the diameter of the wheel.
The middle number is the "aspect ratio", which is the height of the sidewall expressed as a percentage of the tire width. In other words, a 245/50 tire has a sidewall that is about 5" tall at proper inflation.
When you go to an 18" wheel you subtract one full inch of sidewall height so instead of spongy rubber compounds you have aluminum (or steel, eek) which doesn't absorb shock as well. That is terribly oversimplifying things, but it would take several pages to discuss the various parameters.
For more fun with tire sizes, check places like Tire size calculator and www.1010tires.com
Take care and enjoy the ride,
Greg
After the wheels is tires: I would not exceed the proper size on the tire. I'd be shocked if you could go to that size and not rub on the front.
FWIW, here's what the tire numbers mean. And it's both in answer to your question and you'll shortly understand why.
The first number is the cross-sectional width at the tread expressed in MM, so a 245 is about ten inches wide.
The third number is the diameter of the wheel.
The middle number is the "aspect ratio", which is the height of the sidewall expressed as a percentage of the tire width. In other words, a 245/50 tire has a sidewall that is about 5" tall at proper inflation.
When you go to an 18" wheel you subtract one full inch of sidewall height so instead of spongy rubber compounds you have aluminum (or steel, eek) which doesn't absorb shock as well. That is terribly oversimplifying things, but it would take several pages to discuss the various parameters.
For more fun with tire sizes, check places like Tire size calculator and www.1010tires.com
Take care and enjoy the ride,
Greg
