Find out whatever the most common tire size is and preselect it. This serves both to explain how it works (i.e. you have to make 3 selections) and lets people see what the site does with just one click (instead of 4).
Also, while "no images" may be special on HN, users don't give a shit. Images used right are enormously useful. In this case, I'd add exactly three, symbolizing width, profile and size. Also, separate the three selections a bit more from each other, and perhaps even add a stylized tire to the top/logo to make it immediately clear what the site is about.
Images, just like any other UI paradigm, have their appropriate and inappropriate uses. Producing a UI with no images, and shining the spotlight on that fact, is just the same as saying "Look at this UI with no buttons!", which is silly if buttons might be appropriate, just as images could be appropriate in this case.