I have no idea what size the tires on my car are. But I do know the make, model (and year) of my car. I'd rather enter these details into the size, and the site could tell me what tyre size is recommended, and as part of that do a search for the prices.
For the US market, there's at least one company that provides this data. Unfortunately, I can't remember the name, sorry. :(
I do know it exists, though. I used to work for a major American tire manufacturer and maintained a couple of versions of their tire selector application. They would send over a huge file (I think a spreadsheet) with a combination of every year/make/model/trim and its associated OEM tire sizes. I think we also sent over a list of our tires (models and sizes) and they would include that in the db for us. I believe it was left to us to import and normalize it, which was kind of a pain in its own right.
Car make / model - there's only a finite set of these. Take a look at the categories on Autotrader or something similar.
Tire Sizes - ask your local mechanic how they would find this out? There's probably a catalogue they would use.
I have no idea what size the tires on my car are. But I do know the make, model (and year) of my car. I'd rather enter these details into the size, and the site could tell me what tyre size is recommended, and as part of that do a search for the prices.