A wheel's spec line, like 18×9.5 ET35 5×114.3, tells you almost everything about whether it will fit. Here's how to read each part.
Diameter and width: 18×9.5
The first number is the diameter in inches, measured where the tire sits (the bead seat), not across the outer lip. The second is the width in inches, also measured between the bead seats. A wheel's outer edge is roughly an inch wider than its listed width.
The diameter decides which tires fit: an 18-inch wheel takes tires ending in R18, such as 245/40R18. Every tire also has an approved range of wheel widths. A 245-wide tire usually fits wheels from about 8 to 9.5 inches wide, but check the exact range for your tire.
Offset: ET35
Offset is the distance in millimeters from the wheel's centerline to the face that bolts to the car. "ET" comes from the German Einpresstiefe.
- Positive offset (ET35): the mounting face is toward the outside, so the wheel sits further inside the fender. Most modern cars use positive offset.
- Zero offset (ET0): the mounting face is exactly in the middle.
- Negative offset (ET-12): the mounting face is toward the inside, so the wheel pokes out. Common on lifted trucks.
Lowering the offset by 10 mm pushes the wheel 10 mm further out. Going wider moves both edges: an extra inch of width adds about half an inch on each side at the same offset.
Backspacing
Truck wheels are often listed by backspacing: the distance from the mounting face to the wheel's inner edge, in inches. To convert: backspacing ≈ (width + 1) ÷ 2 + offset in mm ÷ 25.4. An 18×9 ET0 wheel has about 5 inches of backspacing.
Bolt pattern: 5×114.3
The first number is how many lug bolts there are; the second is the diameter of the circle they sit on, in millimeters. 5×114.3 is the same as 5×4.5 inches. The bolt pattern must match your car exactly. Some wheels are drilled for two patterns, shown as 5×127/5×139.7.
Center bore
The hole in the middle must be the same size as your car's hub or larger. If it's larger, a hub-centric ring fills the gap so the wheel centers on the hub rather than only on the lug nuts, which helps prevent vibration.
Checking fit before you buy
- Match the bolt pattern exactly, and the center bore or use rings.
- Stay close to your car's original offset unless you know you have room, and allow for brake caliper clearance, especially with big brakes.
- Check the tire's approved wheel-width range.
- Look up what other owners of your car run successfully; the vehicle pages list wheels in typical sizes for popular cars.