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The understanding of offset can be a bit tricky, but with some images for all you visual learners and some good description, we can get a good understanding of what offset is. Let's dive right in.
Offset is the distance between the center of the rim and the mounting surface of the rim where it sits on the hub of the car (see image below). Offset is typically measured in millimeters, so you have to be careful when reading numbers that look something like "ET +35". Most of the time, you will see the offset written something like "ET +35mm" which the "mm" clearly indicates millimeters. As a rule of thumb, the higher the offset the more the wheel will sit in almost like the wheel is tucked in. On the contrary, the lower the offset the more the wheel will stick out. Offset sizes can range anywhere between a high offset (i.e. +50mm) to even a negative offset (i.e. -20mm), which in this case means the wheels will stick out and give that "deep dish" look, where the lip of the wheels sticks out further than the spokes.
If you understood what offset is, you’ll be able to understand backspacing very easily. Backspacing is pretty much the measurement from the backside hub of the wheel to the rear end, up until the width of the wheel (opposite side of the spokes, also see the same image above). Measuring the backspacing is relatively easier than measuring offset, although they both measure the wheel positioning. The best way to measure backspacing is the following:
The best way to measure the offset is the following using this formula 25.4 x (backspacing -.5 x full rim width):
To give you an example, let's assume I have a wheel with 11 inches wide and the backspacing is 7 inches, which would make the formula look like this:
25.4 x (7 - .5 x 11)
My offset, in this case, would be: 38.1mm
Be very careful when trying to figure out what is the best offset and backspacing needed for your car. Some cars may have bigger calipers than other cars so just because one wheel may fit on a certain car within a certain year, does not necessarily mean it will fit in the same car but newer year. We always encourage our customers to call us or email us inquiring about fitment first before placing your order. We hope this helped clarify and bring a better understanding of how offset and backspacing works along with understanding how to measure them.