Dumb Question..what is back-space
maybe this will help
How to Measure Wheel BackSpace
Items required to measure wheel backspace: 
Tape measureStraight edgeWheel w/o tire (preferred)The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace
maybe this will help
How to Measure Wheel BackSpace
Items required to measure wheel backspace: Tape measureStraight edgeWheel w/o tire (preferred)The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace