If you're considering upgrading your car or truck with larger diameter wheels, you've come to the right place. When doing research for your purchase you may encounter terms that you may not be familiar with like offset, centerbore and bolt pattern. These are wheel dimensions and this article was prepared to explain the various dimensions and how they are measured. To get the proper wheel for your car or truck, the wheel must have the right combination of these dimensions. Just as the sleeve length on a shirt or the inseam on a pair of pants has to be right for them to fit your body, your wheels have to be the right size for them to fit properly on your car or truck.
Diameter is the most familiar wheel dimension: When someone says they have 20" rims, they're referring to the wheel diameter. Diameter is the distance between the bead seating areas (where the tire seals to the rim) at the opposite sides of the rim when viewed from the side, not including the wheel flanges. The wheel diameter determines how much of the wheel that you see when you look at a car or truck; the larger the diameter the more wheel that is visible.
It may be hard to believe but it wasn't so long ago that 14" and 15" rims were the standard sizes for most cars, whether these wheels were on new cars from the automobile manufacturers or from the aftermarket. Today, 16" and 17" rims are standard on most ordinary cars, with 19" and 20" sizes on performance models. And in the aftermarket, the sky's the limit, with wheel sizes up to 30" and beyond. When you want to go to a larger diameter wheel, the easiest way is to do so while still maintaining the same approximate tire diameter. Many systems on your car like the driveline gearing, speedometer, anti-lock brake system and stability control system were designed to work with a particular tire diameter. Significantly altering this diameter can negatively affect vehicle performance and the function of these systems. So, in order to maintain the same approximate tire diameter, the sidewall of the tire must become shorter as the wheel diameter is increased. This concept is called "Plus-Sizing", with plus one meaning going up one size in wheel diameter, plus two going up to the next size, and so on.
Plus-Sizing works as follows: A tire's sidewall height is known as the aspect ratio and it's expressed as a percentage of the width of the tire. An aspect ratio of 55 means that the sidewall height is 55% of the tire width. As the wheel diameter is increased, the aspect ratio is reduced so the tire diameter remains approximately the same. For example, a 205mm wide tire with a 55 aspect ratio on a 16" rim, commonly expressed as 205/55R16, can be replaced with a 215/45R17 (plus one) or 225/40R18 tire (plus two) and rim combination and the overall tire diameter will stay within a few tenths of an inch.
This graphic illustrates the concept of plus-sizing.
Even though the diameter of the wheel increases, the overall tire diameter remains approximately the same.
Besides seeing more beautiful rim and less boring black tire, another advantage to larger diameter wheels is that your car may handle better. The shorter sidewalls of low profile tires are stiffer and don’t flex as much. Larger wheels tend to be heavier, which may make steering feel a little sluggish, but many aftermarket wheels can be larger in diameter and still be lighter than factory wheels. Larger diameter wheels are usually wider as well, enabling wider tires to be mounted, so there’s more rubber on the road for increased traction.
Width is the distance between the bead seating areas across the wheel rim. This dimension has also increased as tires have become wider. However, the most important consideration for wheel width is that it be the proper size for the tires that will be installed. Every tire manufacturer specifies a range of rim widths for each tire size, but even within this range, the mounted width of the tire will change. The actual mounted tire width can increase by almost an inch in some cases when going from the narrowest to the widest rim specified. This may not sound like much but it may be critical when selecting a tire and wheel package. A tire mounted on a narrower rim within the range may fit on the vehicle, where the same tire on a wider rim might not.
Offset and Back Spacing
Offset is the distance between the wheel's hub mounting surface (where the wheel bolts to the vehicle) and the wheel's true centerline. On a wheel with zero offset, the hub mounting surface is at the wheel centerline. If the hub mounting surface is toward the wheel face (the visible side of the wheel) side of the centerline, then the offset is said to be positive. This configuration places more of the wheel over the hub and brake components, and is common on front wheel drive cars. Negative offset means the hub mounting surface is closer to the back side of the wheel, so more of the wheel extends outward. So called "deep dish" wheels have a negative offset.
Back space is the distance between the hub mounting surface and the inner edge of the wheel rim. The offset and back space must be correct on a replacement wheel to ensure the wheel and tire assembly fits properly in the wheel wel l and doesn't contact the fender, frame or any other components throughout its range of motion.
To assist in understanding how offset affects a wheel’s fitment on a vehicle, the following diagram shows a wheel of the same size in three different offsets on a vehicle that would call for a 0.
The centerbore is the machined hole in the center of the wheel. Ideally it's sized to fit over the flange that extends from the wheel hub. This flange centers the wheel on the hub as the lugs are tightened to prevent wheel runout, which can cause vibration. However, the size of the hub flange is not standard and varies according to vehicle manufacturer. In order to make a wheel that will fit as many different vehicles as possible, many aftermarket wheel manufacturers machine the centerbore larger to use a centering ring. Using the proper size centering ring will reduce the centerbore for a specific vehicle, so it can be centered on the flange. This allows the same wheel to be used on multiple vehicles.
Some wheels are not designed to center on the hub flange. Such wheels are called non-hub centric or lug centric, because they are centered as the lug nuts are tightened. Lug centric wheels should never be tightened with the weight of the vehicle on the wheel, to reduce the risk of runout and vibration.
Wheels are secured to the hub with lug nuts that fasten to studs on the hub, or with bolts that thread into holes in the hub. There can be anywhere from 4 to 8 studs/holes on late model cars and light trucks. Unfortunately the numbers of studs or holes is not the only difference between vehicles, as the distance between the adjacent studs or holes can also vary. This is why a wheel with 5 holes will not fit every vehicle with 5 studs. Bolt pattern is expressed as the number of studs or holes x the distance between 2 specified studs/holes. For example, the bolt pattern 4 x 4.5 means 4 studs/holes with 4.5" between the centers of the studs/holes opposite one another.
It's easy to measure the pattern on vehicles with 4, 6 or 8 studs/holes, just measure between the centers of the studs opposite each other. Five lug patterns are more difficult, as even measuring as indicated in the illustration may not give the exact dimension. All wheel manufacturers have the bolt pattern information for every vehicle they make wheels for, so as long as they know the make, model and year they can supply a wheel with the right pattern.