Josh wrote:

The car is front wheel drive. If you air the tires down a little, the contact patch gets larger and you theoretically have a bit more traction. If you air the tire down too far, it will start to cup in towards the center, which lessens the size of the contact patch, and works against you. It's kind of a balance when it comes to street tires and PSI.

Pumping up the rear tires to a higher pressure makes them roll easier. They will be harder, and maybe even bow out slightly in the center which puts less rubber on the road which equals less rolling resistance.