Believe it or not, all of this wheel fitment malarkey is actually quite straighforward. Some measurements are normally metric and some are often imperial, which can cause issues, but fear not. Our guide makes it all easy:


PCD


Put simply, PCD stands for 'Pitch Circle Diameter', which denotes the bolt pattern. 


If you were to draw to a circle through the bolt holes, the diameter of this circle would be the PCD. 


PCD is written as (for example) 4x100 (4 bolts in a 100mm circle),5x120 (5 bolts in a 120mm circle), 6x130 (6 bolts in a 130mm circle) and so on. 


Mk1/2/2.5 MX5s came with 4x100 PCD wheels. The MK3 shifted up to 5x100, and then came back down to 4x100 for the Mk4.


 

Offset

Offset is a bit trickier.


Offset is the number which determines how far from the hub the outer lip of the rim. Looking at the wheel as a cross section (like below), it is easier to understand offset. If you draw an exact line half way down the centre width of the wheel, the offset is the distance between the mounting face (where the wheel mounts to the hub) and the line you have just drawn.


A higher offset pulls the wheel further into the arch. Too high an offset and the wheel may foul the inner wing. Too low and the wheel may rub on the arches.


Most modern vehicles have an offset which sits between the 18-52 range (thereabouts). Offset can normally be found on the back of a wheel spoke, inside the rim or beneath the centre cap. 


Offset has 'ET' before it, eg. ET20, ET52 or ET0.


Over the years, wheel offset has, on the whole, increased. This has been to allow for a more central position of the hub in relation the tyre's contact with the floor. Especially for the front wheels this results in a more positive handling attitude with less effort required to make the wheels turn.

 


Lower offset wheels are popular because they give a more aggressive look to the vehicle, a look which is often combined with rolled or flared arches to accommodate them. 


MX5s have a normal offset range of 35-45. OEM wheels are usually 38-40. Lower offset wheels may fit without any arch work, however be mindful of rim width, explained below.

 

Rim Width


Rim width should be quoted in straightforward inches, although it is often mistakely quoted with a 'J' on the end. This measurement is the rim contour and is more to do with tyre fitting, however rim width is often mistakenly written as '6.5J', '8J', '9J' etc.


Fitting wider rims may mean that you need to roll your arches or even cut them out and fit overfenders.


Standard MX5 Mk1/2/2.5 wheels are usually in the region of 6.5-7 inches wide.


If you know the dimensions of your wheels and are wondering how a different set would fit, use this excellent resource to help you work out if a new set will fit.