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Street Race Truck More... The Toolbox

Wheel Offset vs Backspacing Explained

9/21/2018

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.

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.

Measuring backspacing is simple. Lay a straight edge across the back of the wheel, then measure down to the face where the wheel bolts to the hub.

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.

This illustration shows the cross-section of a wheel with negative, zero, and positive offsets.

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.

Other Measurements

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.

This is the best way to measure bolt circle without a template. Measure from the outside of one stud to the center of the stud across from it.

There are also plenty of bolt circle templates available, such as our part # 91082027. These make it easy to measure 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, these Wheel Fit kits are handy to have because they allow you to actually install the tires on the car to check clearance. If you haven't purchased tires yet, the Percy's Wheelrite can be a good way to estimate what sizes will fit.

Shop our extensive inventory of wheels here: Wheels

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