Regardless of whether you are replacing a missing hub cap or are buying a shiny new set of alloys for your car, it is important to make sure you get the wheel hub size correct. Measuring the rim with a tape measure will not give you an accurate reading, as the wheel hub measurement does not include the bead of the tire. Using this method will give you a measurement of between one and two inches more than the actually wheel hub size. Tire manufactures are usually pretty helpful with this and print the size on the tire wall itself.
Look at the side wall of the tire. There will be a series of letters and numbers, along with the manufacturer's name.
Locate the last set of numbers starting with an R, which stands for radial.
Read the two numbers after the R. These correspond to the measurement in inches of the wheel hub size and are called the Rim Diameter Code. If the wheels are standard on the car, the numbers should be between 14 and 18. If the numbers have rubbed off slightly due to wear and tear, you may need to use a magnifying glass to read them.
Purchase a new set of hub caps or alloys which have the corresponding size online or from your local auto parts store.
Measuring with a tape measure is not impossible, but it will give you a measurement of a bit over an inch more. If you are fitting alloys, you can fit larger wheels. This will involve buying new tires as well. Check with your mechanic about how large a set of wheels will be able to fit on your car.