Wheel Offset

Offset (stamped "ET" on European wheels, from the German Einpresstiefe) is the distance from the wheel's centreline to its mounting face. Positive offset puts the mounting face toward the street side, so the wheel sits further into the arch. Zero offset puts it dead centre. Negative offset pushes the whole wheel outward.

Measured from the hub face, the two lips of a wheel of width $W$ and offset $ET$ sit at:

Compare old and new to see where the metal moves:

• Width is quoted in inches, so multiply by 25.4 before mixing it with offset in millimetres.
• A positive $\Delta_{\text{outer}}$ means more poke toward the fender lip.
• A positive $\Delta_{\text{inner}}$ means the barrel moves closer to the strut, spring perch or inner arch.

Backspacing

American wheel makers often quote backspacing instead: the distance from the mounting face to the back edge of the wheel, including roughly half an inch of flange each side.