Wheel fitment explained

Reading a wheel spec

A wheel spec looks like 19x9.5 ET35 5x112 CB66.6. That is a 19-inch diameter, 9.5 inches wide, 35 mm offset, five bolts on a 112 mm circle and a 66.6 mm centre bore. Each number does a different job.

Diameter

The wheel's size in inches, from 17 to 22 for our wheels. Going bigger looks sportier but needs a lower-profile tyre to keep the overall diameter the same, and a lower profile rides firmer. Your brakes also set a minimum size, since the wheel has to fit over the calipers.

Width

Measured in inches across the bead seats, where the tyre sits. A wider wheel lets you run a wider tyre and fill the arch, but it has to clear the suspension on the inside and the arch on the outside. Every extra half inch of width adds about 6.35 mm on each side if the offset stays the same.

Offset (ET)

Offset is the distance in millimetres from the wheel's centre line to the face that bolts onto the hub. It is written as ET, from the German Einpresstiefe.

• A higher offset (say ET45) pulls the wheel further into the arch.
• A lower offset (say ET25) pushes it further out.
• Each 1 mm less offset moves the wheel's outer face out by 1 mm.

Flush fitment comes down to offset: the outer edge of the wheel and tyre lines up with the arch, with no gap and no part sticking out. Width and offset are worked out together.

Bolt pattern (PCD)

The number of bolts and the diameter of the circle they sit on, such as 5x112 or 5x120. It must match your hub exactly. Carmakers change it between generations; for example, newer G-series BMWs use 5x112 while older F-series use 5x120.

Centre bore

The hole in the middle of the wheel that locates it on the hub. A custom forged wheel can be machined to your hub's exact size, so the wheel centres properly without hub rings.

Tyre size

A tyre size like 255/35 R19 means 255 mm wide, a sidewall 35% as tall as the width, on a 19-inch wheel. When you change wheel size, pick a tyre that keeps the overall diameter close to factory. That keeps your speedometer accurate and the ride and handling as intended.

Staggered vs square

A staggered setup has wider (and sometimes larger) rear wheels than fronts, as on many BMW M cars and Porsches. A square setup uses the same size all round, so you can rotate your tyres. Both can be made to sit flush.

Brake clearance and load rating

The spokes need enough room to clear your calipers, especially with large performance or ceramic brakes. The wheel also needs a load rating that suits your car's weight. This matters for heavier cars such as large SUVs, MPVs and EVs.

How we work out fitment

Send us your car's model and year, your current wheel and tyre sizes, your brakes and your ride height. We work out diameter, width, offset, bolt pattern and centre bore for a flush fit, and send you the spec to approve before anything is made.

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