Wheel Offset Comparator

What wheel offset actually means

Offset, marked as ET on the back of a wheel, is the distance in millimetres between the wheel's centreline and its hub mounting face. Positive offset means the mounting face sits toward the street side of centre, tucking the wheel inward. Lower or negative offset pushes the wheel outward. Because the hub face is the fixed reference (it bolts to the car in the same place regardless of wheel), any change in width or ET moves the wheel's two edges relative to everything around them: the arch and bodywork on the outside, the strut and suspension arms on the inside.

Poke and backspacing

Two terms describe the same geometry from different sides. Poke is how far the outer lip moves toward or past the arch, the figure that determines whether wheels fill the arches or stick out of them. Backspacing, more common in the US, measures from the inner lip to the mounting face and matters for clearance to the strut and brakes. This tool reports both, because a wheel that looks perfect from the kerb can still be a millimetre from rubbing a coil spring on the inside.

Width and offset interact

A common mistake is comparing ET numbers alone. Going from an 8J ET45 to a 9J ET45 keeps the same offset but the extra inch of width splits evenly: 12.7 mm further out and 12.7 mm further in. That's why a wider wheel usually needs a different ET to sit where you want it. As a rule of thumb, every 0.5J of extra width adds 6.35 mm to each side, and every 1 mm drop in ET moves the whole wheel 1 mm outward.

How much change is safe

There's no single legal limit, but practical guidance: outward movement under about 5 mm is rarely noticeable; 5 to 15 mm usually fits but deserves a check for arch clearance at full lock and full compression, especially with a wider tyre; beyond 15 mm you're into fender-roll and arch-liner territory on most cars. Inward movement toward the suspension is less forgiving. Many cars have less than 10 mm of spare clearance at the strut, so treat any inward change over 5 mm as something to physically measure first. Large offset changes also alter scrub radius, which can add steering kickback and uneven tyre wear.

What this tool doesn't check

This calculator works from nominal rim width, measured between the bead seats, so the true outside faces sit slightly wider once flanges are included. It also can't see your tyre choice (a stretched or wider tyre changes where rubbing happens first), brake caliper clearance behind the spokes, or hub bore and bolt pattern compatibility. Use the millimetre figures here to shortlist, then confirm against your car.