Wheel Offset vs Backspacing Explained
We all know that choosing the right wheel is an important part of any car project. A big part of what makes a wheel "right" is the way it positions the tire in the wheel well. Too far in and it hits the frame, too far out and it hits the fender. We've all been there.
There are two common ways to measure this, offset and backspacing. These two terms are often used interchangeably to discuss the relationship between the rim and the hub where the wheel mounts to the car, but they are definitely not the same thing. We're going to discuss this here, with some illustrations, to help make sense of it all, but first we want to share some important wheel measurement definitions to help you with your wheel fitment:
- Overall Wheel Width: This is the total width outside of the wheel lip flange, usually 1-inch wider than the inside “stated width” wheel lip measurement the tire seats. This is measured without a tire mounted using a ruler or measuring tape across the wheel and is expressed in inches.
- Wheel Centerline: This is the overall wheel width measurement divided by 2 to halve the dimension, giving us the true centerline dimension of the wheel, expressed in inches.
- Backspace: This is simply the distance from the wheel mounting face (where it sits on your brake rotor or drum) to the rear outside wheel lip flange. This is measured using a straight edge across the back of the wheel (without a tire mounted helps) and a ruler or measuring tape from the mounting face to the straight edge and is also expressed in inches.
- Offset: This is the measurement between the wheel mounting face and the centerline of the wheel’s width. This one is a little harder to measure, especially on wheels that are solid or have small window openings between the spokes, and confusing enough to some it is expressed in millimeters that can be either a positive or negative dimension.
How to Measure Wheel Backspacing?
This is the one that most of us are probably used to seeing when looking at wheels for our hot rods and muscle cars. Quite simply, it's the distance from the back of the rim to the mounting hub at the center of the wheel. With the wheel face down, throw a straight edge across the back of the rim, measure down to the center with a tape measure and you have your backspace.
An alternative backspacing measurement style some people use is to lay the wheel face up and measure through the wheel's center hole with a tape measure or ruler from the floor surface to the backside of the wheel face. This is more commonly seen with steel wheels, as they have a larger center opening, but it can be useful for other wheel designs or as a way to double check your measurements taken with the wheel facing down as shown above.
What is Wheel Offset?
We seem to encounter offset more often when we're looking at wheels for late-model cars. This one's a little trickier to measure, but it's easy to understand. Typically expressed in millimeters, offset refers to the distance between the wheel's mounting hub and the center line of the wheel. The diagram below will make more sense of this.
As you can see, this distance can be either negative or positive. High positive offset wheels are typically found on front-wheel-drive cars and modern rear-wheel-drive performance cars such as Corvettes, Camaros, and Mustangs. Negative offsets yield "deep-dish" wheels commonly found on some of our favorite older cars. So, if a wheel has an offset of 50mm, that means that the face where the wheel will bolt to the hub is 50 millimeters to the outer, curb side from the wheel's centerline. Conversely, an offset of -50mm is the opposite, 50 millimeters to the inside of the centerline, yielding a deeper "dish" in the wheel.
When bolted up to your car this means that a lower offset wheel will stick out farther. If your car was originally equipped with a +50mm offset and you replace it with a +25mm wheel, the wheel will stick out approximately one inch farther. Obviously, a negative offset works the same way. A -25mm wheel replaced with a -50mm wheel will move 1" outward as well.
An Alternative Way to Determine Wheel Offset
Offset is the hardest wheel measurement to determine, so using the two easier to obtain dimensions of overall wheel width and backspacing we can then determine the wheel’s offset.
Let's use a 10-inch overall wheel width. This makes our wheel centerline dimension to be 5 inches (10 divided by 2= 5). Let's say your backspacing measurement is 5-3/4-inches. At this point we’ve taken two simple measurements and we have everything we need to calculate offset with some simple math.
Formula: Backspace – wheel centerline = offset
Any fractional answer (the 3/4 in the backspace) needs to be converted to a decimal equivalent by division first.
3 divided by 4 = .750
Our wheel sample: 5.750 – 5 = .750-inch
However, since offset is stated in millimeters, we have one more calculation to run. There are 25.4 millimeters in an inch, so multiply your answer from your first formula by 25.4 to obtain your offset in millimeters.
.750 x 25.4 = 19.05mm positive offset (wheel mounting surface towards the outside of the wheel)
If the backspace measurement is smaller than the wheel centerline measurement your offset would be negative (wheel mounting surface towards the inside of the wheel).
While the math isn't that hard, an excellent way to determine the correct offset and/or backspacing for your vehicle is to use the actual tires you plan to run and verify their fitment in your wheel wells with a wheel fitment simulator tool like the one shown here.
What Other Wheel Measurements Are Important?
Also worth noting are bolt circle and center bore. Bolt circle refers to the diameter of a circle drawn through the center of the wheel studs. On a four or six-lug car, this is easy to measure, simply go center to center on the holes or studs. Five lug wheels and hubs are more common to the cars that we love, and the odd number can make them a bit tricky to measure. To quickly measure on the wheel, center the measuring tape or ruler on one hole, skip a hole, then measure to the outside of the next hole.
There are also plenty of bolt circle templates available to help with this measurement. These make it easy to measure your bolt circle quickly and accurately.
Also critical is the center bore. This is the hole through the center of the wheel, and if it's smaller than the protruding part of your hub or axle, the wheel will not fit. Check out our Wheel Bolt Pattern Guide to learn more about measuring all types of wheel bolt patterns.
In Conclusion
The most important takeaway here is not to confuse offset and backspacing. With that established, you can make an informed decision when selecting aftermarket wheels for your car. Don't forget that there are tools available that can help you reach a decision. If you have your tires already, our wheel fitment simulator previously mentioned is handy to have because it allows you to actually install the tires on the car to check clearance.
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Updated by Mark Houlahan
