Just picked up some extra winter rims and tires.

I need to buy some nuts, what size are they?

 

found my own answer. M12 by 1.5
This means The metric size gives both the diameter and Pitch as Maa x bb where aa is the diameter in millimeters (mm) and bb is the Pitch - the distance in millimeters (mm) between threads. So M12 x 1.5 has a 12mm diameter and a pitch of 1.5 mm.