Offset and Backspacing
When you bought your vehicle from the dealership, engineers have put long hard hours to calculate a wheel and tire combo that would work well with the vehicle. Calculating width, diameter, offset, rear spacing, offset, and much more. Most people think; "The bolt patterns are the same, let's bolt them up." That is where people finally find a problem. Some may fit, some may not, and some will look unsightly. Anyhow, those engineers designed a wheel that would distribute load evenly on the wheel bearings, clear the rotors and braking system parts, and also keep you from rubbing your frame or front bumper. So, well, how do you figure out which works best and what is what - just tell me what I need and move on; right? Well, hopefully this thread will clear up some issues on wheels and how important some aspects of wheels are.
Backspacing is the distance from the inside bolting surface of the rim to the outer edge of the inboard side of the rim. Offset is the distance from the exact center of the rim to the outer edges of the rim. Positive offset means that the center of the wheel is moved towards the vehicle, while negative offset means the center of the wheel is moved away from the vehicle. Some images on the bottom will help figure this out.
In most cases, the factory wheels have positive offset (especially the new Explorers). Now most aftermarket wheels will have negative offset to push the wheel out further - reason being is to clear bigger tires and to keep you away from hitting frame, bumper, fender well, etc with your tires. The bad part is that there will be more load on the lugs, bearings, spindles, and even cause a larger turning radius. Also the tires will go past the fender well, which in some states can be illegal. Another good thing about negative offset besides clearing bigger tires is the imroved stability with being wider.
Measuring Offset and Backspacing:
Backspacing is easy to measure. Place a straight edge across the wheel like above and then measure from the bottom of the straight edge to the mounting pad of the wheel. This will give you your backspacing measurments.
To calculate offset is even easier, measure the wheel's overall width, subtract its backspace measurement, and divide that by two. You're done.
That picture is of my newest wheels (15x8s) with 3-3/4" backspacing and -19mm offset. That picture demonstrates how far the wheel is pushed out with that set up.
That picture is of the oldest setup which is 15x8s with 3.25" backspacing and -30 offset.
So if the backspacing is 3.25" and the offset is -30mm (-1.18 inches) and your wheel width is 8" then you have 3.57" of wheel that will stick out from where it mounts. It is a little complicated to understand, but it's simple once you do get the hang of it. Refer to the bottom for a picture. It is a rough estimate of what the measurments would look like with a wheel.
Use this to calculate MM to Inches and vise versa
NEW: Offset Calculator
Hope this helps a lot of people out.