The wheel size for a motor vehicle or similar wheel has a number of parameters.
Units The millimetre is most commonly used to specify dimensions in modern production, but marketing of wheel sizes towards customers is still sometimes done with traditional systems. For example, wheels for road bicycles are often referred to as 700C, when they actually measure 622 mm. Wheel diameters and widths for cars are stated in inches, while car tire bead diameters are stated in inches and widths are in millimetres.
Wheel size
The wheel size is the size designation of a wheel given by its diameter, width, and offset. The diameter of the wheel is the diameter of the cylindrical surface on which the tire bead rides. The width is the inside distance between the bead seat faces. The offset is the distance from the wheel's true centerline (half the width) to the wheel's mounting surface. Offset is covered in more detail below. A typical wheel size will be listed beginning with the diameter, then the width, and lastly the offset (+ or - for positive or negative). Although wheel sizes are marketed with measurements in inches, the Michelin TRX introduced in 1975 was marketed in millimeters. For example, 17 × 8.5 × +35 designates a diameter of 17 inches, width of 8.5 inches, and +35 mm positive offset (432 × 216 × +35 in fully metric numbers). Replacing the wheels on a car with larger ones can involve using tires with a lower profile. This is done to keep the overall diameter of the tire the same as stock to ensure the same clearances are achieved. Larger wheels are typically desired for their appearance but could also offer more space for brake components. This can come at cost of performance though as larger wheels can weigh more. Alternatively, smaller wheels are chosen to fit a specific style of vehicle. An example of this is the Lowrider Culture in which smaller wheels are largely desired. Wheels can be widened to allow for a wider tire to be used and to poke the wheel out to the fender of the vehicle. Running a wider tire allows for more of the vehicle's power to be put to the ground because there is a larger surface area making contact with the road. This will improve a vehicle's performance when it comes to acceleration, handling, and braking.
Bolt pattern
The bolt pattern determines the number and position of the bolt holes on the wheel to line up with your vehicle's studs on the mounting hubs. The bolt holes are spaced evenly about the bolt hole circle. Wheel studs are the bolts that are on your mounting hub and are used along with lug nuts to attach the wheel to the car. The bolt hole circle is the circle that the center of each bolt aligns with. The second number in a bolt pattern is the diameter of this circle. The bolt circle has the same center point as the mounting hub to ensure that the wheel will be concentric with the mounting hub. The bolt circle's measurement is called the bolt circle diameter (BCD), also called the pitch circle diameter (PCD). The bolt circle diameter is typically expressed in mm and accompanies the number of bolts in your vehicle's bolt pattern. One example of a common bolt pattern is 5x100 mm. This means there are 5 bolts evenly spaced about a 100 mm bolt circle. The picture to the right is an example of a 5×100 mm bolt pattern on a Subaru BRZ. The wheel has 5 lug nuts and utilizes a 100 mm bolt circle diameter.
Some of the most common BCD values are 100 mm (≈3.94 inches), 112 mm (≈4.41 inches), and 114.3 mm (4.5 inches). Always check your owner's manual or call your local car dealership to confirm the bolt pattern on your vehicle. Over the years, over 30 different bolt patterns have been used by car manufacturers, with most of the different bolt patterns being incompatible with each other.
Lug nuts and wheel studs vs. bolts On vehicles with wheel studs, wheels must be fitted with the correct type of lug nuts. On vehicles without wheel studs, wheels must be fitted with the correct type of lug bolts. Lug nuts (or bolts) will have either flat, tapered (conical), or ball (radius) seats. The type of seat a wheel requires will determine the appropriate lug nuts required to securely attach the wheel to the vehicle. A flat seat type has a flat end that puts pressure on the wheel and compress it against the mounting hub. Similarly, tapered and ball seat types have a conical or semicircular end, respectively. A place to find the lug nut type is to check OEM (Original Equipment Manufacturer) specifications if you have stock wheels or contact the wheel manufacturer if you have aftermarket wheels. Some aftermarket wheels will only fit smaller lug nuts, or not allow an ordinary lug nut to be properly torqued down because a socket will not fit into the lug hole. Tuner lug nuts were created to solve this problem by utilizing a special key to allow removal and installation with standard lug wrench or socket. The design of tuner lug nuts can range from bit style to multisided or spline drive, and are sometimes lightweight for performance purposes. Another variation of lug nut is the "locking wheel nut", which is used as a theft prevention method to keep thieves from stealing a vehicle's wheels. When utilizing locking wheel nuts, one standard lug nut on each wheel is replaced with a nut that requires a unique key (typically a computer-designed, rounded star shape) to fit and remove the nut. This ensures that at least one lug nut will remain attached and, in theory, should prevent theft. However, universal removal tools are available which grip the head of the locking nut using a hardened left-hand thread. The success of locking wheel nuts depends on the determination of the would-be thief and the tools that they have available to them.
Offset
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