These cast resin wheel inserts are for the Pendle Slot Racing 13" alloy wheels and have an external diameter of 10mm.
 
They have been made to closley resemble the wheel used on the Project Binky YouTube build by Bad Obsession Motorsport, and were used on my SuperBinky 1/32 Scalextric Mini build, see link to the right of these words.
 
Set of 4.