 Here is a good example of...  Here is a good example of how important correct back spacing is. The car above is the same body style and has the same 275/60R15 tire mounted on a 10-inch-wide rim as the one below. The wheel above, however, has 5 1/2 inches of backspacing (4 1/2 inches below), which centers it in the wheelwell and gives it an additional inch or so of clearance at the wheel opening. This is the difference between fitting properly, and rubbing the fender's wheel opening like the tire below. |
 |
 If you already have your wheels,...  If you already have your wheels, you can place them on the car and use the wheels as your measuring guide. Again, use a straight edge on the inner and outer wheel lip, and then a tape measure at the desired tire's maximum diameter to check for interference. |
 Measuring for front tire and...  Measuring for front tire and wheel fitment is similar to the rear with the added dimension of the steering. Multiple measurements must be taken with the steering turned fully in each direction to ensure no interference will occur. Be sure to consider suspension travel as well when taking your measurements. |
Load Index and Speed Rating
Two more important considerations when considering tires are the load index and the speed rating of the tires. The load index quite simply refers to how much weight a tire can carry. Some tires have the load index in pounds stamped into the side of the tire, but most have a coded load index. Most tire manufacturers have a load index chart posted on their web site that will give a numerical value, in pounds, for the load index of the tire. Once the value, in pounds, is determined, simply multiply that number by four to get the maximum load-carrying capacity of your set of tires. It is never a good idea to put tires on your car that have a lower load index than the original equipment.
The speed rating given a particular tire is issued by the U.S. government and is based on how well a tire reaches and sustained a specific speed. Tires with a higher speed rating will generally provide better high-speed handling, so if better cornering response is desired try a tire with a higher speed rating. Tires with a "Z" speed rating (the highest speed rating) will be indicated as stated in the P-metric coding. Tires with a lower than "Z" rating will be indicated in the tire's service description, which is a series of letters/numbers immediately following the P-metric coding. For example, a tire with a "S" speed rating may be coded as: P275/60R15 91S, the S being the speed rating of the tire. Always remember a tire's speed rating depicts the speed at which it is tested, not the recommended or continuous speed limitation of the tire. Also, never mix tires of different speed ratings on your vehicle as serious handling problems could result. For a complete list of alphabetic speed ratings and their corresponding speeds please check the sidebar in this article.
Temperature Grade
The temperature grade of a tire is also indicated on the tire's sidewall and is a letter designation. This grade represents the tire's resistance to heat when tested under controlled conditions. Temperature grades from highest to lowest are A, B, and C. All three grades do pass federal safety standards, but a tire with an "A" temperature grade will run the coolest while performing its job.
Traction Grade
This rating represents a tire's ability to stop on wet pavement as measured under controlled test conditions on both concrete and asphalt surfaces. The traction grades, from highest to lowest, are A, B, and C. Remember these grades do not represent a tire's ability to corner or to hold power upon acceleration, just their ability to stop under controlled conditions.
Treadwear Grade
The treadwear grade is a comparative rating based on the wear of a tire when tested carefully under controlled conditions. This method of grading is only useful when comparing tires made by the same manufacturer since different manufacturers use different methods to determine treadwear. Theoretically, a tire with a 300-treadwear grade should last twice a long as a tire graded 150, so long as they are both manufactured by the same company.