Sorex tyres range from 20, softest through to 40, hardest compound. Generally use 24 or 28 indoors on carpet, and for cool weather outdoors on tarmac. Use 32 for warm weather and 36 for hot weather, and 40 for nitro touring cars. 36 chosen as control tyre for EFRA 2008 Euros. 28, 32 and 36 allowed for UK BTCC championship.