If you are new to automotive wheel sizing the specifications might look like an Algebra problem. By the end of this article you will be able to decode wheel specifications like a pro and understand exactly what you need to look for to fit wheels on your car. For the purposes of the article we will use a real life example of a set of Volk Racing TE37 in 18x10J +25 5x120 72.6mm CB as found on an E90 M3.
Wheel Diameter
Let’s start with one of the easiest dimensions to understand that still has a couple of caveats. If you have an 18x10J wheel it represents an 18” outer diameter and it requires an 18” tire. If you take a tape measure to the outside of your wheel, however, you will measure a larger number than 18 inches. This is because the diameter is measured from the tire bead seat on the barrel to the tire bead seat on the other side of the circle. For most wheels the flange height is approximately 17.5mm, so our measured height from rim to rim will be 18” + 2x17.5mm or about 492mm.
The other note is that wheel diameter is always measured in inches, even in countries that use the metric system. Why? To make it extra confusing. You will need to do these conversions between millimeters and inches constantly when dealing with wheel measurements.
Wheel Width
The second number in a wheel size is the width. Like wheel diameter, if you take a tape measure from rim to rim this will not match the specified dimension. This is measured from inner bead seat to outer bead seat. The flange width from the outside of the rim to the bead seat will be about 11.0mm for most wheels.
Our 18x10J Volk TE37SLs are 10 inches wide. Wheel widths are typically expressed in half-inch increments: 9, 9.5, 10, 10.5, etc. Occasionally you will find a unique size like an 8.25” or 9.75”. This is less common and usually means a wheel was designed for a specific platform and fitment. Take for instance the Titan7 T-S5 that comes in an 18x10.7”. This wheel was designed specifically for the MKV Toyota Supra. It is a very nice flex that they have sought the ultimate fitment for the platform.
The wheel width determines what tire sizes will fit properly on the wheel. Too narrow of a tire on a wide wheel creates a “stretched fitment” while too wide of a tire creates a “ballooned” fitment. While both can be done intentionally for aesthetic or performance reasons, there are recommended tire width ranges for each wheel width and each tire manufacturer will recommend a wheel width for their specific tire in a specific size. As a general rule, you want to match your tire section width (in inches) to be within 1 to 2 inches of your wheel width. For our 10” wide wheel, tire widths from 245mm (~9.6”) to 295mm (~11.6”) might be appropriate. Our setup runs a 275mm tire on all four corners.
Flange Profile aka “J”
You might have noticed the “J” in our 18x10J wheel specification. This letter refers to the flange shape we mentioned above. The flange profile is defined by the Society of Automotive Engineers (SAE) and affects how the tire sits on the wheel.
Common flange profile designations include:
- J: The most common profile for modern passenger cars. Found on the vast majority of street wheels.
- JJ: Slightly different profile geometry, common on SUVs and some European vehicles.
- K: Rare, found on some older wheels.
- B, P, D: Mostly obsolete profiles from vintage wheels.
For most people, the flange profile can be ignored since modern tires are designed to work with J and JJ profiles interchangeably. The flange specification is mostly used to show off to your car friends when you say you’re choosing between a “10J or a 10.5J”.
Offset and Backspacing
We now find ourselves at the dimension that causes the most confusion amongst enthusiasts. Offset is the distance in millimeters from the wheel’s mounting surface to the centerline of the wheel.
Offset is often expressed as ET followed by a number, or sometimes just the number with a + or - sign. ET stands for “Einpresstiefe,” which is German for “insertion depth.” This measurement system was standardized by the Germans and they got it to spread.
Our TE37s have an ET25 or +25mm offset. This means the mounting surface is 25 millimeters towards the outside of the wheel from the center line. There are three types of offset:
Positive Offset: The mounting surface is towards the outside of the wheel. This is the most common type of offset and is found on the vast majority of modern vehicles. For the same wheel width a higher positive offset value will move the wheel more inward and closer to suspension components. A lower positive offset will push the wheels outward closer to the fenders.
Zero Offset: The mounting surface is exactly at the centerline of the wheel. This is relatively rare for modern street cars.
Negative Offset: The mounting surface is toward the inside (brake side) of the wheel. This pushes the wheel outboard. This is what creates “dish” or concavity in the wheel face. This aggressive appearance is common on lifted trucks, some classic muscle cars, and modern widebody builds. Negative offset is expressed with a minus sign, like ET-12 or -12mm offset.
Offset Matters
It’s worth noting that offset directly affects how much “dish” or concavity a wheel has. Lower offset numbers (whether less positive or more negative) create more dish, while higher positive offsets create flatter wheel faces. For example, an 18x10 ET45 wheel will have a much flatter face than an 18x10 ET25 wheel of the same design. This is why enthusiasts often seek lower offsets, not just for fitment, but for the aggressive deep-dish aesthetic.
Beyond aesthetics, offset determines where your wheel sits in relation to your fender and suspension components. Too low of an offset can cause rubbing on the fender, while too high of an offset will cause interference with suspension components or brake calipers. Offset also affects scrub radius and steering feel.
Backspacing
While offset is the European standard measurement, backspacing is the “American” equivalent. Backspacing is the distance in inches from the mounting surface to the inner edge of the wheel barrel. You can convert between offset and backspacing with the following formula:
Backspacing = (Wheel Width / 2) + (Offset / 25.4)
For our 10” wide wheel with +25mm offset:
Backspacing = (10 / 2) + (25 / 25.4) = 5” + 0.98” = 5.98”
Most modern wheel manufacturers use offset rather than backspacing, but you’ll still see backspacing referenced in truck and off-road communities. The backspacing measurement is particularly useful when checking for inner fender clearance.
Bolt Pattern or PCD
The bolt pattern, also known as Pitch Circle Diameter or PCD, tells you how many lug holes the wheel has and the diameter of the circle they form. This is expressed as the number of lugs followed by the diameter in millimeters.
Your bolt pattern must match exactly. Unlike offset where you have some flexibility, a wheel with the wrong bolt pattern simply will not fit. Some wheels are dual-drilled with two sets of bolt holes to fit multiple patterns (like 5x100/5x114.3), but these require longer lug bolts and may have aesthetic compromises due to the extra holes.
If you need to measure your bolt pattern: for even-lug wheels (4, 6, 8), measure straight across center-to-center. For odd-lug wheels (5, or the rare 7), measure from the center of one hole to the outer edge of the hole two positions away.
Centerbore or Hub Bore
The center bore is the diameter of the hole in the center of the wheel that fits over your vehicle’s hub. This measurement is in millimeters and needs to match or be larger than your vehicle’s hub diameter.
Our example TE37s have a 72.6mm center bore, which matches the E90 M3’s hub diameter exactly. This fitment is called hub-centric fitment: the wheel is centered and supported by the hub, with the lug bolts simply clamping it in place. This ensures perfect centering and reduces vibration.
Many aftermarket wheels have a larger center bore to fit multiple vehicles. For example, a wheel might have a 73mm center bore to fit various applications. In this case, you would use hub-centric rings to adapt the larger wheel bore to your vehicle’s smaller hub. These inexpensive plastic or aluminum rings fill the gap and ensure proper hub-centric fitment.
The alternative is lug-centric fitment, where the center bore is larger than the hub and the wheel is centered purely by the lug bolts. This is less ideal and can lead to vibration if not perfectly installed or torqued properly. Always use hub-centric rings when your wheel’s center bore is larger than your vehicle’s hub diameter. They are cheap insurance against vibration and uneven tire wear.
Load Rating
A wheel’s load rating indicates the maximum weight each wheel can safely support. This is especially important for trucks, SUVs, and performance cars.
Wheel load ratings are typically expressed in pounds or kilograms per wheel. To determine the appropriate load rating for your vehicle, you need to know your vehicle’s Gross Vehicle Weight Rating (GVWR). This is the maximum total weight your vehicle is designed to carry including passengers and cargo. This can be found on a sticker inside the driver’s door jamb or in your owner’s manual.
Divide your GVWR by four to get the minimum load rating per wheel. However, it’s important to consider that weight is not always distributed evenly as front and rear axles often carry different loads. Your vehicle’s door jamb sticker will show front and rear axle weights. Use the heavier axle weight, divide by two, and ensure your wheels can handle at least that much weight per wheel.
For passenger car wheels, load ratings are often not stamped on the wheel itself and must be found in the manufacturer’s technical specifications. Always ensure your wheels meet or exceed your vehicle’s weight requirements. Installing wheels with insufficient load ratings is dangerous and can lead to premature wheel failure.
Wheel Markings and Certifications
When you look at a wheel, you will likely see various markings stamped into the barrel or behind the spokes. These include size specifications, manufacturing information, and safety certifications.
Common certifications include:
JWL (Japanese Light Alloy Wheel): Japanese testing standard for passenger cars. JWL-T is the variant for trucks.
VIA: Vehicle Inspection Association certification from Japan. This is a voluntary standard with more rigorous testing than JWL and is highly regarded.
TÜV: German certification indicating the wheel has passed TÜV testing standards. Highly regarded in the European market.
DOT or SAE: Indicates compliance with Department of Transportation or Society of Automotive Engineers standards for the US market.
Reputable wheel manufacturers will have at least one of these certifications stamped on their wheels. If a wheel has no certifications whatsoever, proceed with caution as it may not have undergone proper safety testing.
Phew
You did it. You got to the end. Amazing. Now you can decode wheel specs like a pro. Have a burning question you want answered in a deep dive guide? Email us at hello@rimlist.com.