Tread width and rim size seem fairly straightforward. What do the aspect ratio and speed rating mean?
The aspect ratio is a way of describing the sidewall height, or the height which the tread sits off of the rim. Rather than give a simple length measurement, it is expressed as a percentage of the tire section width. In the above example, the “50” means the sidewall is 50% of the maximum width of the tire, or 112.5mm (about 4.4 inches). A lower aspect ratio means a lower “profile” tire, which generally means a stiffer sidewall and firmer cornering, sometimes at the expense of ride comfort. People often speak of tires with the same aspect ratios as a “series” of tire, for example, “50-series tires” means tires with a 50 aspect ratio. Performance tires are also given a speed rating to give people some idea of how the tires were constructed and how far they can be pushed. The table below translates speed rating to design maximum mph.
