I don't get how a 255/45-18 tire would fit on a 17x8 wheel lol.
Let me break it down:
17x8 = 245/45-17 (25.68")
17x9 = 275/40-17 (25.66")
17x10.5= 315/35-17 (25.68")
18x8 = 245/40-18 (25.72")
18x9 = 275/35-18 (25.58")
18x10 = 295/35-18 (26.13")
The number in ( ) is the overall dire diameter, notice how the numbers change it stays relatively the same? That is GOOD, unless you are specifically looking for a larger overall diameter, but you should only be looking for that for specific performance purposes.
All of the widths are +/- 10mm since 1 inch = 25mm...... rule of thumb = for every inch you go wider you add 30mm. As for the sidewall, that is kind of flexible, its ok at times to go with 5% higher on the sidewall, but don't go 5% lower on the sidewalls or you will have a shorter tire than stock.
If you want to keep the stock height those numbers are what you want. Some (including scotty) would go as far to say that the 275s are too wide for a 9" wheel, i agree, a 265 is a better choice, but you have to take into account the overall diameter, so i would only go with that specific choice up front.
Take for example my next set up. I will be using 265/35-18s up front and 295/35-18s in back. That is 25.3" up front and 26.13" in back, but i Autocross and will be getting into open course racing so i have it mapped out better for my application.
Also different tire manufacturers do make different widths, BFG generally runs wider than normal and Nitto runs more narrow than normal, depends on the tire though, the next tire i am getting is a BFG and the 295 version of it fits perfect on a 10".
Summary: If you want aftermarket wheels and tires that will fit. Look at the different sizes i posted and match them to your wheel. Realistically for most people the sizes being 10mm wide or narrow won't affect the car at all.