However, there can sometimes be a clearance issue on earlier cars, particularly TR2-3A where it in not uncommon to find the body far from central on the chassis, leaving lots of clearance (especially at the back) one side and a possible interference in bump (when the wheel goes up into the wheel arch), between the tyre and the lip of the wing. For this reason it is important to understand that the extra width of the wider two of the three wheels is not spaced equally about the centre line of the inner hub. This variation is identified by measuring the offset which is explained as follows.

The offset (sometimes called backspace) is the measurement from the inner face of the hub (where the spokes attach and not where the splined hub attaches) and the inner lip of the rim (not the face where the tyre sits but the turned over edge). This is not ideal as the edges being used to measure are not machined and are sometimes rough. However, this method is practical for measuring at home without the benefit of complex measuring instruments. The table below shows how much of the additional width is added to the inboard and outboard for the two wider sizes.

WW450P/C 4" wide. Back space 83mm
WW452P/C 4½" wide. Back space 87mm. Extra 4mm inboard, extra 9mm outboard
WW4572P/C 5½" wide. Back space 95.5mm. Extra 12.5mm inboard, extra 25.5mm outboard

For reference Steel and Aluminium wheels can be measured more accurately and the measurement is quote as the German ET (Einpresstiefe or, literally, press depth) which is the measurement from the mounting face to the centre line of the wheel.