=Brainstorming technical problems= 

i think that is exactly what u said..., but u 4get that the car is not 
touching the surface, but its tires do. so basically if u enlarge the tires 
they have more surface... which means more surface touching the floor. think 
about doing the opposite surely u dont think that a car with tires that thin 
will hold the car. also when turning there is the so called "imaginery" or 
not centrifugal force so basically the car wants 2 flip over 2 the side, and 
has more grip because it is wide...its like women who have high heel 
shoes...y do they trip over ? samething. they r not stable

anyway when they taught u in school, they generelized and simplified the 
surface and made it 1. but actually the surface has many friction "spots" 
(not the real term) so as larger the surface is there r many friction spots 
that u make the calculation -->

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->as u can c the 
larger surface has more friction spots, ofcourse in reality
 there a lot more...