I agree with basically everything that has been said so far. Now, I don't drive or buy tires, but I do dabble in design, so here are the first things I see when I start nit-picking:

* The nav text is too small to read comfortably.

* For the nav in the top right: why is it in that spot? It doesn't line up with other content, so it looks out of place.

* TyreSum.beta – my first thought was: 'there is a .beta TLD?' This is confusing.

* The logotype doesn't need an underline when you mouse over it.

* 'We compare so you don't have to' – you have used periods throughout your whole site... but not to end your slogan?

* Why are the buttons shiny? Then, why aren't they shiny when you click on them? Why is the gradient on the properties-of-tyres buttons is different from the submit button's gradient?

* Why do the buttons have squared corners when the container and the submit button have rounded ones? (Or, why do the
other things have rounded corners?)

* Why are the buttons all square, when the text takes up less than half of their height? Since they are, why is the text (very tight!) at the top?

* 'Width', 'Profile', and 'Size' look awkwardly narrow centred on the wide rows of buttons. You could make them look chunkier, or move them to the left of the rows, or something. It would also be nice to see more padding between them and the rows above. The padding on the page is not very consistent all-around.

* For example… the padding on the container is awkward. There is a bunch of whitespace on the left and right, and an
awkward mismatch of whitespace size on top and bottom.

* The container has a shadow but nothing else does; except for the gradients on the buttons, if you see that as a shadow – in which case this is doubly awkward because the shadows are running in different directions.

* Why is the container 940px wide? Since the contents don't seem to fill it horizontally, it could be narrower. This would be good because it would work better on smaller screens – especially mobile devices.

* On the search page:

* It's a little strange that 'make and cheapest model' contains links, and 'website' doesn't. I couldn't guess where those links were going to take me.

* You have a 'price' column, and then a 'purchase' column, where each button has the price on it. The 'price' column could go. (You have to be able to sort based on the purchase column, then, though.)

* Why are the contents of some columns centred while others are left aligned?

* Why are the website names a different size and font weight, and why are they all in caps?

* As others mentioned, you don't need the back buttons.

* 'Showing…' doesn't line up with anything.

* I don't have a suggestion here per se, but it takes several clicks to get from stating a size to buying a tyre. To me, the information hierarchy was not obvious. After I got past the search page, I was confused each time I saw another link. I couldn't predict where any of those would take me. Eventually I figured it out, but it would be good if I didn't have to.

* Write good, semantic HTML, and validate it. You are using a bunch of deprecated elements (like <center>) where you should be using CSS instead, for example.
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.tyresum.c...

Hope that helps. Basically, I think their are two main points. The first is about implementation: you should try to follow coding best practices, like using semantic HTML and sanitizing your SQL. The second, which relates to design, is that every aspect of your design should have a 'why.' If someone asks (or, hopefully, you ask yourself) 'why is this here? why is this this colour? why is this this size?' etc. you should have a rock-solid answer. If the user finds themselves wondering those things, it probably means it needs work; likewise if you don't have an answer.