Mechanics will oftentimes need to measure the offset of a wheel when changing tire sizes. This is the measurement is defined as the side to side positioning of the wheel. This is important because an improper offset could lead to contact between tires and other parts of the vehicle. Determine the offset of a wheel and whether this offset is a negative or positive value.
Determine the wheel backspace measurement. This is done by laying the wheel face down so the back is facing up towards you. Lay your ruler across the inboard flange. Take the measuring tape and measure the distance from the rule to the hub mounting pad in inches. Your result is the wheel backspace.
Measure the width of the rim. Do this by measuring from the outboard flange to the inboard flange. Write your answer down in inches.
Calculate the center line of the wheel. This is accomplished by taking the width of rim and dividing the number by two. For example if your total rim width was six inches, the center line will be three inches.
Subtract the center line from the backspace to find the offset. If the center line was three inches and the backspace was two inches the offset would be negative one inch.
Use your answer to determine of the offset is positive or negative. If you your center line is greater than your backspace you will have a negative offset. If the center line is less than your backspace you will have a positive offset.
