Brakes

[Midmichigangal]

I am wondering how much of a gap there is so suppose to be between the wheel and the brake pads? I noticed that my rear brake pad is rubbing right against the wheel. Does having the brake rub against the wheel slow down the bike?

[PinkBike]

oh yes, it will really slow you down or make it harder to pedal (it happened to lance in the tour once).
how much room is really personal preference, depending on feel at the lever.
try pulling the brake away from the rim (they sometimes shift a little if they're jostled) or loosen your skewer and make sure your back wheel is centered in the pads then retighten the skewer.

[OakLeaf]

If it's only rubbing at one point in a wheel revolution, then the wheel needs to be trued. Otherwise, what she said.

[susan.wells]

There is a centering screw to center the caliper to the wheel. You can refer to the Park Tool Website for details. Scroll down to Pad and Caliper Adjustment. Points 5 and 6 provide details and photos for this adjustment. I'm assuming you're refering to dual pivot brakes. The Park Tool Website has a wealth of information along with photos which I have found to be very helpful.