Tires are round usually black and are supposed to hold air. This is the extent of many drivers knowledge about tires. I have to admit my knowledge of tires is not extensive. In all the years I owned shops we never actively pursued tire sales, it never seemed like we could make a profit, they were dirty heavy work, and there was more profit in other areas such as drivability and electrical problems which other shops didn’t want to tackle. Tires come in directional, position numerous wear, traction and speed ratings. It can be confusing and tires are not a frequent purchase so there is not a lot of experience on the buyer’s part. If the shop you frequent sells tires this is the place to go as you already have a history with them, if not ask them co-workers and friends where they go. Understanding the sidewall of a tire is not hard if you break it downSize is simple just copy it down off your old tire there are several tire conversions charts on the net but be careful they can lead you to a wrong tire for your vehicle quite easily. This can also cause speedometer problems, so I recommend you stick with the tire size that was original on your vehicle. Load index and speed ratings should also be close to original unless you have a large truck or a fast sports car. Load index’s go from 71-761 Lbs to 110-1060Lbs. Speed from L-75 MPH to V-140MPH most common is S&T 112-118 MPH. There is also a W&V rating for exotic sport cars. Tire pressure listed on the tire is not the recommended pressure but the maximum pressure allowed. The recommended is either on the drivers or glove box door. This is usually slightly on the low side to improve ride. I usually go a few pounds over but you have to be careful as tire pressure will increase 1# for every 10% of temperature or up to 4# on a hot highway. Tire pressure can change up to 5# as the seasons change and 5# can cause problems with handling, traction and tire life. A tire in the hot sun can reach 135%. Tire size is measured in MM. The width of the tire is the first number measured from sidewall to sidewall, the second the ratio of height to width or a P205/50R17 the ratio would be 50% OF 205 and this would be the tire height. If you have some spare time inflate your tire to this height and that would equate to the proper air pressure. Have tried it with some success but out temperatures here vary too much so I try to hit a happy medium. When you buy new tires check the pressure they are usually set low to provide a better ride. Two other markings on the tire are wear and traction. These are on a sliding scale the better traction the softer the tire and reduced tire life the less traction and improved tread life. Tires should be replaced or checked by a reputable tire dealer every 5 years; storage conditions can change this figure drastically. Sun is a tires worst enemy with heat coming in a close second. One last hint do not use tire dressing as this will increase tire checking and cracking. I do use it but the car I use it on is always stored inside also I do not rotate I know I should but it never seems to get on my priority list and I feel by the time I pay to rotate for the life of the tire I can almost buy new tires.