It is necessary to check the tyre size as some makes and models eg Corolla from Toyota has been manufactured with tyre sizes (and rims) ranging from 13" to 15"..
To check the tyre size to make sure you get the correct hubcap delivered first time.. record the code included in the tyre mould by the manufacture during production - codes vary from brand to brand some common formats are SR 185-15 or R 205-16.
If you do not have a code easily visible and imprinted on your tyres you may need to consult a tyre fitter to help determine what size rims you have and which hubcaps you will need....
The numbers we need are the " -15" section as this indicates that the rim is (in this case) 15" in the picture below the rim size is (To be added)....
The actual outer dimensions for hubcaps can vary from manufacturer to manufacturer...