okay if you look a the tire size on most german cars(Benz, or BMWs) why is it they have different size on the front vs back?
now I understand like drag cars. but every something like a C300 or a Bluetech diesel Benz which isn't like a AMG Benz. why not just put 4 of the "fatter" tires on it?
thanks in advance.

 

Fatter rear tires reduce oversteer. The ultimate example is the Porsche 911's. I once drove an 80's Carrera and it wanted to fishtail while driving straight on the highway. Scary car.

Since you can't rotate them with the fronts, it also means more vi$it$ to the dealer for replacments.

Eventually, the Germans will fully copy out Acura's SH-AWD system and this problem will go away (unless you really want to drag race with the Rustangs and Fire Chickens).

 

^^^ older 911's were notorious for lift off, snap over steer in large part because of the torsion bar suspension and terrible weight distribution. Every generation has gotten better and better and now it is nothing more then a memory.

It typically looks better and makes chassis tuning much easier.

 

also narrower front tires allow for a decreased turning radius.

 

these are rear wheel drive cars and need more grip in the back. the rims are also wider in the back, therefore, u need a thicker tire and an additional side effect of rear wheel drive cars is that the rear tires wear faster than the fronts.

look at a stock g35 coupe with the 19" rays rims for example, the rim width in the front is 8" with a 225/40/19 and the back is a 8.5" with something like a 255/40/19 (can't remember the rear tire sizes off the top of my head).