Sumitomo Tire Replacement
When should I replace my tires?
While at first a ±2% diameter increase or reduction in tire diameter may sound very limiting, in most cases it allows approximately a ±1/2" diameter change.
Additionally to help with the selection of substitute sizes, a system called "Plus Sizing" was developed. Use Plus Sizing to take into account the diameters of the available tires & the wheels, & this helps select the appropriate tire width that ensures adequate load capacity. Maintaining the tire's overall diameter helps maintain accurate speed data going into the computer.
How many tires do I need?
Since tires affect the personality & performance of your vehicle, all four tires should be as identical as possible or handling problems may arise. If your tires don't match, it is possible that one end of your vehicle won't respond as quickly or completely as the other, making it more difficult to control.
Just one tire?
If your tires have a lot of remaining tread depth, but you need to replace just one that has been damaged by an accident, road hazard or a vandal, you should replace it with a tire that exactly matches the others. Select a replacement tire of the same brand, line, size & speed rating. While there may be a less expensive tire available, it wouldn't be a bargain this time because it would be different than the other three tires on your vehicle.
A pair of tires?
If two of your tires have a lot of remaining tread depth, but you need to replace the other two because they were damaged or have worn out, you should replace them with a pair of tires that come as close as possible to matching your existing tires. While identical new tires are desirable, others of the same size & type can also provide good results. Only consider selecting new tires that are from the same tire category as your existing tires. New tires should be installed on the rear axle.
While your vehicle is being serviced ask your mechanic why one pair of tires have worn faster than the other pair. Was it caused by a lack of tire rotation, out-of-spec wheel alignment or loose mechanical parts? Once the problem has been found, it can be corrected before it damages your new tires. Keep in mind that your ultimate goal is that all of your tires always wear out at the same time so they can be replaced as a set.
A set of tires?
If all of your tires are wearing out together, you have the greatest flexibility in tire selection. If you were happy with the original tires, simply replace them. If you want longer treadwear, a smoother ride or more handling, there are probably tires that will help you accomplish that.
How do I calculate tire dimensions?
Example...185/60R14 85H or 185/60HR14
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 in the example above, the width is 185mm or 7.28".
The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".
The last number is the diameter of the wheel in inches.
To figure the outside diameter of a tire, take the sidewall height & multiply by 2, (remember that the diameter is made up of 2 sidewalls, the one above the wheel, & the one below the wheel) & add the diameter of the wheel to get your answer. 185mm x .60=111mm x 2=222mm + 355.6mm (14") = 577.6mm or 22.74" (((185 X .60) X 2) / 25.4 ) + 14 = 22.74
A shorter formula exists that makes this a little easier. It is Section Width (SW) times the Aspect Ratio (AR) (full number) divided by 1270 (a constant that takes care of the percentages, the two sidewalls, & the conversion to inches), plus the rim diameter (RD) (in inches), which gives the Outside Diameter (OD). Here is how it would look with the same example above.
((SW X AR) / 1270) + RD = OD
((185 X 60) / 1270) + 14 = 22.74 inches