OK you boy racers who like to mod your car explain to me how the offset works on a Mag Wheel ?

I have been told a set of Mag wheels with Winter tyres for a differing car we have moved on are et37 and for a new car we have buying fresh off the shelf it would be et50. Would the et13 difference make the car unroad worthy ?

Thank Yorky. FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current

The offset is the distance of the wheel boss ( the bit that bolts to the hub) is set from the wheel centreline, the et50 would give you more tyre to the inside of the wheel arch by about 1/2 an inch so not unroadworthy as the et13 possibly has a slightly narrower rim if not then it will stick out about 1/2" towards the wing outer edge . http://www.tsw.com/explore/what-is-wheel-offset.php my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

 FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current