Using our experience of selling trailer wheels and tyres on-line we have categorised our wheels by the number of studs and the PCD. The PCD (pitch circle diameter) is the diameter of a theoretical circle drawn through the centre of your wheels bolt holes (shown in red below), this is a crucial measurement and it is important that you get it right. 
 

To work out your PCD see the images below:  

Fig 1. 4 Stud Rim. To work out your PCD measure the distance betwen the centre of stud A to the centre of stud B (ie two opposite studs).

Fig 2. 5 Stud Rim. To work out your PCD measure the distance betwen the centre of stud C to the centre of stud D (ie two adjacent studs) and then multiply by 1.7012, or you can measure the diameter of the theoretical red circle. 

Our most popular wheels and tyres are the 8 & 10" rims with 4" & 115mm PCD, 4" are used on many small UK made trailers and the 115mm's are supplied on the Halfords, Indespension and Maypole type trailers.