Quick answer: A wheel size like 18x9.5 +22 means an 18-inch diameter, a 9.5-inch width and a +22 mm offset. Diameter and width are measured at the bead seats where the tyre sits, not across the outer lips. Offset is the distance in millimetres from the wheel's centreline to its mounting face. To fully describe a fitment you also need the stud pattern (PCD), such as 5x114.3, and the centre bore, which is 73.1 mm on SpeedWorks passenger wheels.
How to read a wheel size
Most wheels have their specs cast or stamped on the back of a spoke or inside the barrel. A typical marking looks like 18x9.5J ET22 5x114.3, and it breaks down like this:
| Marking | Meaning | Example from the SW range |
|---|---|---|
| 18 | Diameter in inches, measured at the bead seat | SW12 is 18-inch |
| 9.5 | Width in inches, between the bead seats | SW12 18x9.5 |
| J | Rim flange profile. J is the standard profile for passenger cars | n/a |
| ET22 or +22 | Offset in mm (ET comes from the German Einpresstiefe) | SW12 18x9.5 +22 |
| 5x114.3 | Stud pattern: 5 studs on a 114.3 mm circle | SW12, SW15, SW18 |
| CB 73.1 | Centre bore diameter in mm | All SW passenger wheels |
Your tyre sidewall tells you the diameter too. In a size like 225/40R18, the "18" means the tyre fits an 18-inch wheel. Your car's tyre placard (usually on the driver's door jamb) lists the factory tyre sizes.
Diameter and width
Wheel diameter and width are nominal figures measured where the tyre bead sits. That's why a tape measure across the face of an 18-inch wheel shows more than 18 inches, and a tape across the back of a 9.5-inch wheel shows more than 9.5 inches. The outer lips (flanges) add to the overall size.
If your wheels aren't marked, measure the overall width lip to lip and allow for the flanges. A tyre shop can confirm the exact figure quickly. Don't guess, as half an inch of width makes a real difference to fitment.
Offset: what it is and how to measure it
Offset is the distance from the centreline of the wheel (halfway across its width) to the face that bolts against the hub.
- Positive offset (+): the mounting face sits towards the outside of the wheel, tucking the wheel further into the car. Most modern cars use positive offset. The SW09 19x8.5 +35 and SW16 19x8.5 +40 are examples.
- Zero offset (0): the mounting face is on the centreline. Our 4x4 SW11 and SW21 are 17x9 with zero offset.
- Lower offset: a smaller number pushes the wheel further out towards the guard. The SW08 15x8 +15 is an example.
Measuring offset at home
- Lay the wheel face down and place a straightedge across the back lip.
- Measure from the straightedge down to the mounting face. This is the backspacing.
- Measure the overall width of the wheel, lip to lip.
- Offset is roughly backspacing minus half the overall width.
For example, a wheel measuring 260 mm overall with 152 mm of backspacing has an offset of about 152 minus 130, or +22 mm. A positive result means positive offset, and a negative result means negative offset.
How width and offset work together
Changing width and offset together moves both the outer face and the inner edge of the wheel. Here's an example using two sizes from our SW06 range:
| Change | Outer face | Inner edge |
|---|---|---|
| 18x8.5 +35 to 18x9.5 +22 | About 25.7 mm further out | About 0.3 mm further from the suspension |
The extra inch of width (25.4 mm) is split evenly, adding 12.7 mm to each side. Dropping the offset by 13 mm then moves the whole wheel 13 mm outward. So nearly all the change shows up at the guard, while the inner clearance barely moves. You can run any comparison like this in our wheel offset calculator.
Real examples: a customer's Nissan Silvia S15 runs the SW06 staggered at 18x8.5 +35 front and 18x9.5 +25 rear. Several S14 and S15 owners run 18x9.5 +22 all round on the SW10 and SW12. Wider, lower-offset setups like these may need guard work or a lowered ride height depending on the car, so ask us before ordering.
Stud pattern (PCD)
Count the studs, then measure the circle they sit on. On four- and six-stud hubs, measure centre to centre across opposite studs. On five-stud hubs, measure from the back edge of one stud to the centre of the stud furthest from it. This gives an approximation. Close patterns like 5x112 and 5x114.3 can't be reliably told apart with a tape, so confirm with your car's spec. Our stud pattern guide covers this in more detail, including which SW wheels are dual-drilled.
Centre bore
Measure across the hole in the middle of the wheel with vernier calipers, and measure the raised spigot on your car's hub the same way. SW passenger wheels have a 73.1 mm bore and the 4x4 SW11 and SW21 have 110.1 mm. If your hub is smaller than the wheel's bore, a hub-centric ring centres the wheel. Our 73.1 mm rings come in inner sizes of 56.1, 60.1, 64.1, 66.1, 66.9, 67.1, 70.3 (plastic only) and 70.7 mm.
Checklist before you buy
- Diameter and width of your current wheels (from the markings or your placard)
- Current offset
- Stud pattern
- Hub centre bore
- Brake caliper size, if upgraded
- Ride height and any guard work already done
Frequently asked questions
Where is the wheel size written on my wheel?
Usually on the back of a spoke, on the inner barrel or near the mounting face. It's visible with the wheel off the car, or sometimes through the spokes. If there's no marking, measure as described above or ask us.
Is a higher or lower offset better?
Neither is better on its own. The right offset depends on your car's hub, suspension, brakes and guards. Lower offsets push the wheel out and higher offsets pull it in. Compare against your current setup with the offset calculator.
What does ET mean on a wheel?
ET is the offset in millimetres. ET22 means +22 mm.
Can you check my measurements?
Yes. Send us your car details and current wheel specs, and we'll suggest SW sizes that suit. You can also browse real cars in our gallery.