Being one of those that have no clue about tyres' sizes, I found the 205/45 17 combination after a few tries. It would be nice if the "tyres size choosing page" would retain your choices, after you get a "Size not found, please try again" or a "Please select Width, Profile and Size".

In the results page you may want to hide the table header's sorting buttons from the "Purchase column" and call the "back" button something like "New search" (this last point is quite debatable of course).

About the UI, it looks good overall (caveat: I have a bias toward non-distracting UIs). I don't really see the need for images.

It would probably be helpful to have a little section telling you how to figure out what size tyres you have. You'd probably need an image, though. See, e.g., http://www.blackcircles.com/tyres.