Finding the Center Line
Place the wheel on the ground with the back side facing up. If the tire is still on your vehicle, then you will need to remove the lug nuts and tire from the vehicle first. First, use a jack or hydraulic lift to get the vehicle off the ground and into a safe position. Then, remove the lug nuts and pull off the tire. Place the tire on the ground with the back of the tire facing upwards.Safety Precaution: Make sure to follow general safety precautions for removing a tire from your car. Turn off the car, wear safety glasses, and disable any automatic start features on the vehicle before you begin.
Lay a straight edge ruler across the center of the tire. Position the straight edge so that it lays flat over the wheel. Ensure the middle of the straight edge is directly over the hub of the wheel. The hub is the metal part in the center of the wheel.
Measure from the floor to the bottom of the straight edge. Place a ruler or tape measure perpendicular to the straight edge near the center of the wheel. Position the straight edge so it is near the center of the hub with 1 end touching the floor under your tire. Then, look at the ruler or measuring tape to find the measurement at the bottom of the straight edge that's laid across the tire. For example, the measurement from the floor to the bottom of the straight edge might be 244 mm (9.6 in). Use a straight edge ruler that has millimeter measurements on it to get the most precise measurement possible. If you don’t have a straight edge with millimeter measurements, multiply the measurement in inches by 25.4 to convert it into millimeters.
Divide the measurement by 2 to find the center line. Take the floor to straight edge measurement and halve to get the measurement for the centerline of the tire. You will need this number to calculate the offset. For example, if the measurement was 244 mm (9.6 in), then the center line measurement is 122 mm (4.8 in).
Calculating the Offset
Measure from the hub to the bottom of the straight edge. Next, reposition the ruler or measuring tape so that 1 end is against the hub in the center of the wheel). Position the other end against the straight edge laying flat across the tire. For example, the distance from the straight edge to the hub might be 172 mm (6.8 in).
Subtract the center line from the hub distance measurement. The resulting number is your offset. Record or remember this number for future reference. For example, if you got 172 mm (6.8 in) for the hub and 122 mm (4.8 in) for the center line, then your final number would be 50 mm (2.0 in). If you got a hub measurement of 100 mm (3.9 in) and a center line measurement of 122 mm (4.8 in), then your final number would be −22 mm (−0.87 in).
Note whether the number is positive, negative, or zero. The offset is considered positive if it’s a positive number, negative if it’s a negative number, and zero if the number is 0. You may need to know the offset of your vehicle’s wheels to ensure that you purchase the correct type of wheels.Tip: The offset is negative if the center line measurement is bigger than the hub measurement. A calculator will also tell you if the result is positive or negative. For example, if you have a front-wheel drive car, then you might need to get positive offset wheels with a specific measurement. Or, you might be interested in getting negative offset wheels or “deep dish” wheels to complement your car’s design, but there may be a limit to how deep you can go according to your car’s specifications.
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