Original equipment manufacturers usually stamp the wheel offset size directly onto the wheel as part of standard wheel markings. If the manufacturer did not stamp the offset size onto the wheel, use a website that determines the standard OEM offset size based on the car model.
The last set of numbers on the rim or wheel marking is the offset size in millimeters. Often the letters "ET" precede the offset size number. For example, a 2014 Acura MDX has wheel markings that read 8Jx18 ET40. Therefore, the offset size is positive 40 millimeters.
An offset size can be positive, negative or zero, because it is the measured distance between the centerline of the wheel and wheel mounting plate. Most wheels, especially OEM wheels, have a positive offset. Only cars with deep-dish rims have negative offsets, and they are usually only available from aftermarket part manufacturers. Not all cars can accommodate this rim style.
Knowing the OEM offset is essential to prevent damage to the vehicle when outfitting a car with custom rims. Custom rims with a significantly smaller offset can cause the wheels to rub against the fenders. Custom rims with a significantly larger offset can rub the brakes or suspension, causing serious damage.