its simple more width, more grip (with exceptions) and from what I understand v8 super cars, f1 and WRC etc run tyres with a stiffer side wall compared to a regular car or sport(road) tyre
Just out of interest in terms of the actual structure of a tyre, how does a semi slick compare to a normal road tyre, do they have stiffer walls, or are they the same, just diffrent compound and tread pattern??
cheers
elliot