Alloy wheel codes are a little more simple than tyres here is the story.
Size: 6,5 x 15 ET38
This is decoded as follows.
6,5 is the width of the rim, again measured in inches between the flanges.
15 is the diameter of the wheel in inches from edge to edge, measured inside the flange with sizes ranging from 10 inch on the original mini's to 23 inch on 4x4 vehicles.
ET38 Another important thing is the wheel offset, which is the distance between the middle line of the centre of the wheel width and the mounting surface of where the wheel bolts to the hub on the vehicle. The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'. Most wheels have a positive offset which means the mounting surface is further outboard than the wheels centre. If you reduce the amount of positive ET on the wheel the the vehicles track is widened and vice versa if the ET number is reduced. Straying too far from the original offset can be detremental as fouling to the suspension and bodywork can occur as well as eccess strain on wheel bolts or studs
