Wow, a easy one. Ok, the reason we use a narrow left rear has 2 reasons. First less rolling resistance. Second, better traction. Why better traction, well if you look at the tire patch of a narrow, you will have a great weight per square inch of surface when compared to a wide tire. Plus, it just looks cool!
A wider tire, will be looser compared to a narrow tire. You really can't compare foams to caps, 2 different worlds. No clue on foam tires, Carpet is for wiping your feet on. I don't have a clue why you would want to run a r/c car on it!
I can't think of a track where a wide left cap tire would be faster. Also with a narrow left rear, you can pull the left side in. This can help the car turn in the middle better and loosen up off the corner. You have many more options in tuning with a narrow left rear.
Narrow lr helps the car turn in the middle ( on caps ) is what I,ve always heard, but Ive never tried a wide lr cap tire :lol: let me know what happens if you try it. I always run full width foams so I can rotate the rear tires and keep tire wear under control mostly, but then again I am a sorry foam tire racer :lol: