Car Wheels

Car wheels come in many sizes depending on the vehicle they are for, from the smaller wheels for Smart Cars, to much larger versions for Hummers.

Modern road tires have several measurements associated with their size as specified by tire codes like P225/70R14. 

The "P" designates the tire as a passenger car tire. There is also an "LT" designation for light trucks. No letter indicates the tire is a European metric tire.c

The first number in the code (e.g. "225") represents the nominal tire width from sidewall to sidewall in millimeters. 

This is followed by the aspect ratio (e.g. "70"), which is the height of the sidewall expressed as a percentage of the section width. 

THe "R" stands for radial and designates a radial tire construction. 

The final number in the code (e.g. "14") is the wheel diameter measured in inches. 

The overall circumference of the tire will increase by increasing any of the tire's specifications.c

For "high flotation" tires, the size is indicated a bit differently, for example 35x1250 R 17/E. The "35" indicates the tire diameter in inches, the "1250" indicates the tire is 12.5" wide, the R indicates radial tire construction, the "17" indicates the wheel diameter in inches and the "E" indicates the number of layers used in the tire's construction, in this case 10; if there is no letter, the tire has the standard 4 plies, while "C" indicates 6, "D" indicates 8 and "E" indicates 10. The higher the number, the stronger the tire.c

The above numbers are followed by the load index and speed rating, usually a number and a letter. With a "92H," you have a tire with a load index of 92, equating to 1,389 pounds of weight the wheel can carry safely.c 

The "H" is the tire's speed rating, given by the U.S. Government to indicate minimum standards for reaching and sustaining a specific speed have been met.c An H means the tire can drive safely up to 130mph. The other speed ratings include:

• Q - up to 100mph
  
• R - up to 106mph
  
• S - up to 112mph
  
• T - up to 118mph
  
• U - up to 124mph
  
• V - up to 149mph
  
• W - up to 168mph
  
• Y - up to 186mph
  
• Z - 149mph and up
  

These markings generally look this this: MAX LOAD 635 kg [1400 lbs] AT 300 kPa [44 psi] MAX PRESS.

• The maximum load limit, in this example "635 kg [1400 lbs]," indicates the tire's maximum load-carrying capabilities when the tire is fully inflated.
  
• The maximum air pressure limit, in this example "300 kPa [44 psi] indicates the maximum operating inflation pressure of the tire.
  

Some tires are marked "Extra Load," "XL" or "RF" for reinforced, denoting the tire's maximum load and air pressure are higher than standard load tires.c

These markings generally look like this: DOT CC9L - YYY1105 - 

• The "DOT" indicates the tire meets the U.S. Department of Transportation's safety standards.
  
• The "CC" identifies the manufacturer of the tire and manufacturing plant.
  
• The "9L" are codes representing tire size.
  
• The "YYY" represents other characteristics of the tire as determined by the manufacturer.
  
• The "11" identifies the week the tire was manufactured.
  
• The "05" identifies the year the tire was manufactured.c
  

These markings generally look like this: TREADWEAR - 520 TRACTION - A TEMPERATURE - A

• The "520"is a treadwear rating that measures the tire's durability. Each tire manufacturer independently determines treadwear, so these numbers and their meanings differ.
  
• The "A" for traction rating indicates the ability of the tire to stop on a straight, wet surface under controlled conditions. Grades include AA, A, B and C, with AA being the highest grade available.
  
• The "A" for temperature rating is a measure of the tire's resistance to heat generation under normal operating conditions. Grades range from A to C with A being the highest.c
  

These markings generally look like this: PLIES: SIDEWALL POLYESTER 1 TREAD POLYESTER 1 + STEEL 2

This indicates what materials were used in the tire's plies and the quantity of each type.c

These markings generally look like this: ROTATION >>--->>

Only some tires indicate the direction of rotation on the sidewall, while other indicate a specific side of the tire intended to face outward from the vehicle. Other tires combine both indications. 

Directional Tires: Directional tires feature arrows on the sidewall that indicate what direction the tire should rotate when the vehicle is moving forward.

Asymmetrical Tires: Asymmetrical tires have the word "outside" labeled on the side of the tire that should face outward from the vehicle.

Directional & Asymmetrical Tires: Tires that are both directional and asymmetrical will indicate what direction the tire must rotate, as well as what side must face outward from the vehicle.c

The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Wheels with the correct centerbore to the car they will be mounted on are known as "hubcentric." Wheels that are not hubcentric are known as "lugcentric," as the job of centering is done by the lug nuts.

Centerbore on aftermarket wheels must be equal to or greater than that of the hub, otherwise the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock or slide into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub.