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Since the dawn of hot rodding, wheels have played a big role in automotive aesthetics. Whether it’s a raked muscle car or a stanced sport compact, the diameter, width, and design of a vehicle’s wheels are often a defining feature. And getting that look right typically requires careful consideration of the wheels’ offset.
Offset is defined as the distance inward or outward of the mounting surface from the centerline of the wheel. This characteristic essentially dictates how close or far away the face of the wheel is from the outside “lip” of the wheel, and adjusting this can create either a flush appearance, or the “deep dish” look that’s often seen on highly customized cars with big, flared-out fenders. But while tweaking a wheel’s offset can help dial in the aesthetic, there’s more to consider than just style.
“Offset also matters when it comes to fitment,” Luke Hayter of Holley Performance explains. “And a number of different factors come into play when it comes to what wheel offset will work in a given car, and what won’t. The width of the axle – along with the shape and width of the wheel wells and fenders – varies from one platform to another. Those characteristics determine what wheel offset you’ll need to ensure that the wheel won’t make contact with chassis components or the bodywork.”
Here are some key terms you should know to help determine the ideal offset for custom wheels. The cutaway-view illustration below shows the difference between the three basic types of wheel offset, and how they can affect suspension clearance and overall fit.
CENTERLINE
As the name implies, the centerline is the point directly in middle of the wheel’s width.
BARREL
Also referred to as the rim, this is the outside ring of the wheel that the tire is mounted to.
ZERO WHEEL OFFSET
This is an offset which is positioned exactly at the wheel’s centerline; the mounting surface is not positioned outward or inward relative to the centerline.
POSITIVE WHEEL OFFSET
This is when the mounting surface is positioned in front of the centerline of the wheel, toward the outside of the car. This creates that flush look that we mentioned earlier, and it’s a common characteristic of the factory wheels on most modern vehicles.
NEGATIVE WHEEL OFFSET
Here the mounting surface is positioned behind the wheel’s centerline (i.e. inward toward the center of the car). This creates the deep-dish look mentioned above. “With trucks it’s common to see a zero offset, or even a negative offset,” Hayter says. “A negative offset means that the mounting face of the wheel is further in on the barrel of the wheel, which creates a situation where more of the barrel of the wheel is pushed outward, giving you a deeper concavity.”
When swapping over to a new set of wheels that differ from your factory wheels in terms of offset and/or width, it’s important to make sure that the offset you choose remains compatible with the vehicle in question. If it isn’t, you’ll immediately run into some serious trouble. “If there’s too much positive offset and the wheel is in too far, it can rub on the inner fender liner – or even suspension components, and things like that,” he says. “On the other side of the coin, if you have too much negative offset, you can run into issues where the wheel makes contact with the fender when the suspension compresses.”
To determine a wheel’s offset, flip the wheel upside down so the face of the wheel is pointed toward the ground. “After that, lay a straight edge across the diameter of the wheel, then place a ruler or a tape measure down to the mounting surface of the wheel – the area that mates with the hub, and you measure that against the total width of the wheel. So if you have a 10-inch-wide wheel, and the measurement from that straight edge you place on top of the wheel to the mounting surface is five inches, you have a zero offset wheel.
If the measurement was six inches, you’d have a wheel with one inch of positive offset. Or if it was four inches, you’d have a wheel with one inch of negative offset. Essentially what you’re doing here is getting the backspace measurement; measuring from the back face of the mounting surface to the back of the wheel is commonly referred to as backspace. And getting that backspace measurement allows us to calculate offset.”
Hayter also notes that offset is typically measured in millimeters, so it’s a good idea to either do your measurements in millimeters from the outset or convert to millimeters after you’ve done the measuring.
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So we now know the backspace and offset of a wheel, but how do we know what offset will work on a particular car? While your mileage may ultimately vary, there are some ways to get a general idea of what will work and what won’t. “The outside is the easy part,” he says. “First, take a wheel off. We want the car to sit at normal ride height, so adjust your jack and stands accordingly to allow for that. Then you just measure from the mating surface on the hub or rotor on the vehicle to the furthest point you can go on the inside edge of the fender. That’s your maximum positive offset, and it that tells you how far out the wheel can come out without potentially making contact with the fender.”
The inside, he tells us, is a little bit trickier. “The rear of the vehicle isn’t so bad – you just measure from the mating surface on the wheel all the way inside to whatever sticks out the furthest, whether that’s the inner fender liner, a brake line, or something else.” He says that although there isn’t exactly science to this, there are ways to ensure that the measurements you’re getting are reliable. “They do make wheel offset tools that you can use to measure all your clearances. It bolts to the hub, just like a wheel would, and it has these different apertures that you can move around to see how far you can go both inward and outward.”
Hayter points out that measuring inward offset clearances at the front of the vehicle adds another variable to the mix. “You can’t just measure from the mating surface inward because the front wheels don’t stay in one static position like the rear wheels do – they turn. Ultimately, factory wheels are usually set in pretty far already, so just make sure that the inside edge is going to be similar to, or further out than the factory stock wheels would be. With the outside edge, you can usually go quite a bit further than the stock wheel. The inside edge is where you’re probably going to be limited beyond the stock wheel’s clearances.”
He also suggests some ways to further ensure that a custom wheel will fit a particular vehicle. “If you’re doing a custom fitment – a situation where you’re not buying something off of the shelf that’s designed to fit a particular vehicle – it’s good practice to put some wheels on the car that you already know will fit properly and then take some measurements with those wheels on it. Turn the steering wheel to full lock in both directions and check out what’s going to end up being the most likely culprit to limit your clearances. Taking some measurements from here should give you a pretty good idea of how much room you have.”
If your outside clearances are borderline and you really want to get a particular wheel offset, there are some potential solutions. This is another area where your mileage may vary, though. “If it’s close, the easiest option is to add some negative camber, if the vehicle allows for that,” Hayter says. “With most vehicles you can adjust that, but with some you can’t do that on the rear, such as vehicles with a solid rear axle. But if the platform allows for it, adding negative camber can definitely help if you’re close to getting it clear.”
Rolling the fenders can also create some additional clearance on the outside edge. When fenders are stamped during manufacturing, the edges of the wheel well openings are pushed inward as the form of the panel is created. Typically, this additional section of metal is left as-is; bending it upward to make it sit parallel with outside of the panel would require an additional step in the manufacturing process that isn’t necessary to ensure that the factory wheels have enough clearance. But when you’re swapping in wider wheels, or wheels with a different offset, rolling that additional material upward can make a substantial difference. “Some high-end vehicles do have the fenders rolled from the factory, but many do not,” says Hayter. “On most there’s a pretty sizable lip on there that you can roll out of the way to give you more clearance.”
If these two options aren’t enough and you need more clearance on the outside edge, you could also consider changing the actual shape of the fender. “Pulling the fender would be the next step,” he says. “And it’s exactly what it sounds like – you’re literally bending the fender outward to give it more clearance. Obviously, if you want that to look good, that needs to be done by a professional – if you just start yanking on the fender, you’re probably going to put a crease in it. And that goes for rolling the fenders, too. You want to make sure you’re having a professional do that, too – or at least use a proper fender rolling tool. Otherwise you may end up damaging the fender in the process.”
If you’re willing to go even further with the body modifications, you can also just integrate fender flares into the bodywork to get that additional clearance. “If you’re in a situation where the wheel is going to make contact with the fender regardless of whether you roll or pull the fender, that would be the other option,” he says. “That would involve trimming the factory fenders and wheel wells, and the flares would cover up the portion that you’re cutting out of the fender and prevent the tires from kicking up debris.”
Hayter also points out that wheel manufacturers and distributors can often help determine what offset will work on a particular vehicle and what will not. “Companies like Rocket Racing Wheels, REV Wheels, and Apex Racing Wheels can be incredibly helpful when it comes to this kind of thing. Also, many tire companies have databases of fitment information that can tell you whether or not that wheel is going to fit your vehicle – and many offer a fitment guarantee. If you’re unsure about measuring this stuff out for yourself, a guaranteed fitment can provide some peace of mind that you’re not going to get stuck with wheels that don’t fit properly.”
