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Message: <blockquote data-quote="Tom328" data-source="post: 739676" data-attributes="member: 16210">First, you need to know what those numbers mean. 255/40 R18 - 255 means 255mm width (section width from the outer to the inner side wall of the tire), the second number is the % of the section width which will represent the height of the sidewall (40% of 255mm = your sidewall height) and 18" for your 18" rims. There will also usually be a number and alphabet following it, e.g 92Y where the number represents the tire load (amount of kg it can take for each tire) and the alphabet represents the speed rating of the tire (maximum speed limit for the tire). Try this link <a href="http://www.tirebuyer.com/education/understanding-tire-sizes-and-types" target="_blank">http://www.tirebuyer.com/education/understanding-tire-sizes-and-types</a>Then there are threadwear ratings, temperature rating, week and year of manufacture etc etc. Using the same rim, you can actually go slightly wider/narrower or taller/shorter. But you also have to make sure that your wheel arch can accomodate whatever tire size you have in mind.As explained, thicker sidewall means more air = more comfort. Wider tire = more contact patch with the road = better grip. Happy hunting!p/s : Google is your best friend!</blockquote>