- Inset and how far the wheel sits out
- Inset (offset) is how many millimetres the mounting face sits outboard of the centreline of the rim. The smaller the number, and the further into negative it goes, the further out the wheel sits. The outer face moves by roughly the same amount as the change in inset: take 10 mm off the inset and the wheel moves about 10 mm outboard. If you change rim width too, half of the extra width is added on each side.
- The rule on poke
- Since the Japanese regulation was amended on 22 June 2017, where the outermost part is the tyre itself, a protrusion of less than 10 mm is permitted within 30 degrees forward and 50 degrees rearward of the wheel centre. This applies only to passenger cars; commercial vehicles on 1- and 4-series plates are not covered even for the tyre. Protrusion of the wheel rim itself is outside this allowance and is never permitted.
- Inboard clearance
- Deciding on the outer face alone is how you end up rubbing on the inside. Increasing inset pulls the wheel inboard and cuts the gap to the strut, suspension arms, brake caliper and tie rod. Check at full lock in both directions and with the suspension at full bump.
- Rim width versus real tyre width
- The actual section width of a tyre depends on the rim it is mounted on. As a rule of thumb, half an inch more rim width gives roughly 5 mm more tyre width. The width in the size marking is the value at the reference rim, so allow for this when you change rim width.
- PCD, bolt count, centre bore and torque
- Even with the outer face right, the wheel will not fit unless PCD (the diameter of the circle through the bolt holes) and the number of holes match. If the centre bore is smaller than the vehicle hub it will not go on; if larger, use hub rings to centre it. After fitting, torque the wheel nuts to the specified figure and re-check them after driving.