Front v. Rear Tire Size Difference = Performance Gain?
 

I notice some folks run wider tires in the rear than in front. What, if any, performance gain is acheived by this set up?

more weight in the back, more grip required
wider tires = more grip

but it will make the car a bit more understeered if the difference is to big
which again could result in snap oversteer and the good old wrong way out exit out of a turn...

it also makes life more difficult if you drive in heavy rain

in a straight line it also means you can get the power down a lot better if one were to do any straight line transmission thrashing...

It also depends on how hard you're leaning on the tires when cornering, and what the suspension settings are. If you're not pushing the existing tires to the limits when cornering, then extra rear grip is purely academic. If the car is balanced and you are currently driving the car at the limits of it's adhesion, then by putting in fatter rear tires in the rear, you'll increase rear grip which will increase understeer. So to really take advantage of the extra grip, you will most likely need to modify your suspension set-up.

If on the other hand your car is already loose at the limit, going to wider tires very often will help, but depending on how wide you go, you may wind up not solving the problem for geometry reasons. So in order to make full use of really wide tires, your car should most likely be equiped with 930 trailing arms and rear suspension, or at least rear camber-boxes.

So the short answer is -- it depends!