Bolt pattern or lug pattern or bolt circle is determined by the number of bolt holes and the diameter of the bolt circle.
Hub Diameter or Center Bore is the large hole at the center of the wheel.
Rear spacing or back spacing is the distance from the backpad to the outside of the rim flange.
Offset: The distance from the center-line of the wheel to the mounting surface of the wheel.
Negative offset: When the back of the back pad is closer to the inside of the wheel; when mounting surface is inboard of the rim center-line.
Positive offset: When the back of the back pad is closer to the street side of the wheel; when the mounting surface is outboard of the rim center-line.
Measure your bolt pattern like you see to the right here.
Measure the backspace of your wheels like this.
First of all it is best to do this without the tire mounted to the wheel. You're going to need a tape measure and also a straight edge that will fit down right inside of the inner flange [see photo at the top of the page for reference]. Lay the wheel face down onto the ground so the backside of the wheel is facing up. Take the straight edge and lay it down across the inner flange of the wheel. Use your tape measure to measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is the wheel's backspace. The photo to the left here shows three wheels with 2",3", and 4" backspace.
Backspace To Offset Conversion Chart
The table on the left is a quick reference for finding offset. Find your rim width and follow the row over to the backspace of your wheel.
Measuring The Wheel Offset Yourself
To calculate the offset you'll need three measurements:
Wheel Center line (Find this by measuring the total distance from outer flange to inner flange and dividing that in half.)
Once you have your measurements, subtract the wheel center line measurement from wheel backspace measurement to get the offset.
If backspace is less than the wheel center-line the offset is negative
If backspace is greater than the wheel center-line the offset is positive
To convert from inches to mm multiply by 25.4
To convert from mm to inches divide by 25.4
Or visit http://www.onlineconversion.com/length_common.htm for help converting.