Complete Wheel Sizing Guide

Basic Wheel Dimensions

The fundamental measurements that define a wheel's size:

Wheel Diameter

Measured in inches, wheel diameter is the distance across the wheel from bead seat to bead seat. Common passenger vehicle sizes range from 15" to 22", while some trucks and SUVs may use even larger diameters. Diameter is always the first number in wheel sizing (e.g., in an 18x8.5 wheel, 18 is the diameter).

When upgrading wheels, it's generally recommended to stay within 2 inches of your vehicle's original equipment size to maintain proper performance characteristics.

Wheel Width

Measured in inches (sometimes in half-inch increments), wheel width is the distance between the inside edges of the wheel flanges where the tire beads seat. This measurement is taken from the inside of the wheel, not the outer visible faces.

Width is the second number in wheel sizing (e.g., in an 18x8.5 wheel, 8.5 is the width in inches). Proper width is crucial for tire fitment—too narrow, and the tire won't seat properly; too wide, and it may cause rubbing issues or improper tire profile.

Lip Size & Style

While not a critical fitment measurement, the lip is the outer edge of the wheel. Lips can vary from minimal (flat face designs) to quite deep (step lip or deep dish wheels). Lip depth is primarily an aesthetic consideration but can affect overall wheel weight and strength.

Offset & Backspacing

These critical measurements determine how the wheel sits in relation to the vehicle's suspension and body:

Offset

Offset is the distance in millimeters from the wheel's mounting surface (where it attaches to the hub) to the centerline of the wheel. It can be:

• Positive Offset: When the mounting surface is toward the front or outside face of the wheel. Most front-wheel-drive vehicles use positive offset wheels.
• Zero Offset: When the mounting surface is even with the centerline of the wheel.
• Negative Offset: When the mounting surface is toward the back or brake side of the wheel. This creates a "deep dish" appearance and pushes the wheel outward from the vehicle.

Offset is typically marked as "ET" followed by a number (e.g., ET45 means +45mm offset). Choosing the correct offset is critical—improper offset can cause rubbing against suspension components, fender interference, and handling issues.

Backspacing

Backspacing is the distance from the mounting surface to the inside edge of the wheel, measured in inches. While offset and backspacing are related measurements, some enthusiasts and manufacturers prefer to use backspacing as it can be easier to visualize how the wheel will fit in the wheel well.

To convert between offset and backspacing:

• Backspacing = (Wheel Width ÷ 2) + (Offset ÷ 25.4)
• Offset = (Backspacing - (Wheel Width ÷ 2)) × 25.4

The 25.4 factor converts between inches (backspacing) and millimeters (offset).

Bolt Pattern

Also called bolt circle or lug pattern, this measurement describes the diameter of the circle formed by the centers of the wheel lugs, and the number of lug holes.

Bolt patterns are expressed as:

• Number of lugs × diameter in millimeters or inches (e.g., 5×114.3mm or 5×4.5")

Common bolt patterns include:

• 4×100mm: Many compact cars and smaller vehicles
• 5×114.3mm (5×4.5"): Widely used on many midsize cars, SUVs, and crossovers
• 6×139.7mm (6×5.5"): Common on trucks and larger SUVs
• 8×165.1mm (8×6.5"): Heavy-duty trucks and commercial vehicles

Bolt patterns must match exactly—even slight variations can prevent proper mounting. While adapters exist, they can introduce additional complexity and potential failure points, so direct fitment is always preferred.

Measuring Bolt Patterns

For even number of lugs (4, 6, 8), measure from center to center of opposite lugs.

For odd number of lugs (5, 7), measure from the center of one lug to the furthest edge of the lug hole that's two positions away, then multiply by the correction factor 1.051 for 5-lug patterns.

Hub Bore & Center Bore

The hub bore (or center bore) is the center hole in the wheel that centers it on the vehicle's hub. This measurement is critical for proper fitment and to prevent vibration issues.

• Hub-centric Fitment: When the wheel's center bore exactly matches the vehicle's hub diameter, creating a precise centering effect.
• Lug-centric Fitment: When the wheel has a larger center bore than the vehicle's hub and relies on the lug nuts to center the wheel.

For wheels with larger center bores than your vehicle's hub, hub-centric rings (spacers that fill the gap between the wheel's center bore and the vehicle's hub) are recommended to achieve proper centering and prevent vibration.

Wheel Construction & Weight

While not directly related to fitment, construction affects a wheel's strength-to-weight ratio, which impacts performance:

• Cast Wheels: Made by pouring molten metal into a mold. Most affordable but typically heavier.
• Flow-Formed/Rotary-Forged Wheels: Cast centers with barrel formed under pressure and heat. Offers improved strength-to-weight ratio over fully cast wheels.
• Forged Wheels: Created by compressing solid metal under extreme pressure. Highest strength-to-weight ratio but most expensive.
• Multi-Piece Wheels: Constructed from separate components (center, inner barrel, outer lip) that are bolted or welded together. Allows for custom widths, offsets, and finishes.

Lighter wheels reduce unsprung weight, improving acceleration, braking, and handling responsiveness. However, wheel strength must always be sufficient for the vehicle's weight and intended use.

Load Rating

Every wheel has a maximum load capacity it can safely support. This rating is particularly important for trucks, SUVs, and vehicles used for towing or hauling.

Wheels must be rated to handle at least the maximum load of the vehicle corner they're mounted on, including any cargo or towing weight. Load ratings are typically cast or stamped on the back of the wheel, often near the valve stem hole.

Never select a wheel with a load rating lower than what your vehicle requires, regardless of aesthetic appeal or other features.

How to Read Wheel Specifications

Wheel specifications are typically formatted as follows:

18x8.5 ET45 5x114.3 72.6

Breaking this down:

• 18: Wheel diameter in inches
• 8.5: Wheel width in inches
• ET45: Offset of +45mm
• 5x114.3: 5-lug bolt pattern with 114.3mm diameter
• 72.6: Center bore diameter in millimeters

Additional specifications might include load rating, pressure rating, and manufacturing information, typically found stamped or cast on the back of the wheel.

How Size Changes Affect Performance

Understanding how wheel size changes impact vehicle performance can help you make informed decisions:

Increasing Diameter

• Benefits: Improved appearance, potentially sharper handling, ability to fit larger brake components
• Drawbacks: Potentially harsher ride, increased weight (unless using lightweight construction), higher cost

Increasing Width

• Benefits: Ability to mount wider tires for increased traction, improved stability
• Drawbacks: Potential fitment issues, increased weight, possible negative effects on steering feel

Changing Offset

• More Positive: Wheels sit further inward, potentially improving clearance for suspension components but possibly creating fender clearance issues
• More Negative: Wheels sit further outward, creating a more aggressive stance but potentially causing rubbing on fenders or suspension components

Any significant change in wheel specifications should be carefully evaluated for its impact on fitment, handling, and overall safety. Our wheel experts can help you determine the ideal specifications for your specific vehicle and performance goals.