Lot of info in the tire/wheel section....
The most common wheel spacer width is 1.5". Using a wheel spacer will make your 5.5" BS rim act like a 4" BS rim and move the rim out to allow a wider tire. If you do not want to buy rims but want to use the factory 5.5" BS rims, you can buy spacers if you have rubbing issues ( not a definite as a 255 is not super wide.) If you are buying new rims, get a 4 or 4.5" BS and call it good. Adding a spacer to a 4" BS rim would be a no-no....probably assure tearing your flares off, at least.
Quality spacers ( spidertrax, for example ) have long histories of being very dependable. Cheap ebay China specials...not so much. Ya gets whats youse payz for...
16 = rim diameter. 2nd number is the width of the rim from bead mounting surface to bead mounting surface-think "from internal lip to internal lip." x8 = 1" wider than x7.