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. This pre-glued 32 is the Official Tyre for the 2009 BRCA British National Championship as voted for by Britains top drivers..