Small tire shop

Hi, I have a small tire shop. I have a website already dktireservice.com and I want to upgrade it. The thing I would like, mainly, is a calulator? where you enter your year/make/model to find out the size that belongs on you car. Mainly trying to make it a little smoother and more interactive. Any ideas that could help me out without having to pay an arm and a leg for the tire size database?