ET stands for "Einpress Tiefe," which is a German phrase that translates to "insertion depth". It is a measurement given in millimeters that indicates the distance between the mounting pad of the wheel and the centerline of the wheel. Wheel offset, or ET, determines how far out or in a set of wheels will sit in a cars wheel arches. The offset is not the only factor responsible for the location of the wheel against the wheel arches, as the width of the wheel also plays a role.

There are three types of wheel offset:

• Zero offset: The hub mounting surface is in line with the centerline of the wheel.
• Positive offset: The hub mounting surface is in front (more toward the street side) of the centerline of the wheel. Most wheels on front-wheel drive cars and newer rear-drive vehicles have positive offset.
• Negative offset: The hub mounting surface is behind the wheel centerline. "Deep dish" wheels are typically a negative offset.

The offset is an important factor to consider when customizing a car with aftermarket wheels and tires. Proper offset assures that the new package has enough clearance so that nothing rubs against the suspension, brakes, or vehicle body, such as fenders, bumpers, and mud flaps. It is also important for driving safety, as the wrong offset can reduce vehicle stability or interfere with braking.