Some retailers may refuse to install tires for one reason, as the tires purchased online were not the same size as the ones that came on her vehicle. This has raised questions about what a tire shop is allowed to do when it comes to installing tires they didn't sell. These types of tires could be worn beyond safe limits, or have manufacturer defects, or not be the correct size for the vehicle. However, liability may be a deciding factor, as large tire shops and corporate chain stores may not be allowed to install these tires. The 3% rule for tire sizing is used to determine which tire size a vehicle is supposed to have, and it's easy to know which size a car should have.