Negative offset is the when the mounting surface is closer to the center of the car compared to the wheel centerline. Positive offset is the opposite, and refers to when the mounting surface of the wheel is farther away from the car than the wheel's centerline. Zero offset refers to a wheel that is neither positive or negative offset. The mounting surface is centered in the wheel.
http://www.ebay.com/gds/How-To-Choose-Wheel-Offset-/10000000001639843/g.html
(Lea en español) The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*. The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.. The hub mounting surface is even with the centerline of the wheel. The hub mounting surface is located in the front half of the wheel closer to the wheel face.
https://www.tirerack.com/wheels/tech/techpage.jsp?techid=101
Positive Offset wheels have their mounting face toward the front face of the wheel. Negative Offset wheels have their mounting face toward the rear of the wheel. First, measure the overall width of the wheel (remember, just because a wheel is 18×7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement between the outboard flange and the inboard flange is 7.5”).
http://www.wheelsrus.com.au/blog/tech/wheel-101-wheel-terminology-explained/
Offset on wheels is basically a measurement on the location of the mounting surface of a wheel, where it bolts to a car, compared to the centerline of the wheel. Offset is a big factor in determining whether or not the shiny new set of wheels you are looking at will fit your car.
http://www.ebay.com/gds/How-To-Choose-Wheel-Offset-/10000000001639843/g.html
What is offset? The offset of a vehicle's wheel is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel. It can thus be either positive or negative, and is typically measured in millimeters.
http://www.wheel-fitment.com/what-is-the-offset/
Negative Offset wheels have their mounting face toward the rear of the wheel. The easiest way calculate wheel offset is as follows: First, measure the overall width of the wheel (remember, just because a wheel is 18×7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement between the outboard flange and the inboard flange is 7.5”). Next, divide that width of the wheel by two; this will give you the centreline of the wheel.
http://www.wheelsrus.com.au/blog/tech/wheel-101-wheel-terminology-explained/
Now, Offset is much the same. Offset is measured in millimeters insteaad of inches like backspacing. Instead of measuring from the mounting pad to the the back edge of the wheel to get the backspacing. This is a little more difficult to do by hand, lets say you have wheel standing up on the floor (No tire on it) if you measure the width of the wheel across, say it's 8.
http://www.ebay.com/gds/Wheel-Offset-Backspacing-what-it-means-finally-/10000000009860383/g.html
Lets start with backspacing on a wheel, this is very simple to measure. Take a straight edge of some sort and lay it across the back of the wheel, make sure if a tire is mounted on the wheel it does not interfear. Then take a ruler or tape measure & measure from the back of the mounting pad (area that has the holes for the wheel studs) to the straight edge.
http://www.ebay.com/gds/Wheel-Offset-Backspacing-what-it-means-finally-/10000000009860383/g.html
Offset has a significant effect on many elements of a vehicle's suspension, including suspension geometry, clearance between the tire and suspension elements, the scrub radius of the steering system, and visually, the width of the wheel faces relative to the car's bodywork. Left: Offset 30, right Offset 40.
http://www.wheel-fitment.com/what-is-the-offset/
That means the spokes sit farther back in the wheels. This offset gives the deep lip look that a lot of people want on their cars. Most of the time people use reverse offset on smaller diameter wheels that will not rub their wheel well, although reverse offset is available up to 22 inch wire wheels. Reverse offset wire wheels are the main wheel used on lowriders.
http://www.ebay.com/gds/How-To-Choose-Wheel-Offset-/10000000001639843/g.html
what is a wheel offset mean
A wheel offset is the distance from its hub mounting surface to the centerline of the wheel. The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.