[Ho Lee Kow]

I was looking for some tyres for different cars. There are measurements like 20x10 I can see but how do I know they will fit on the wheels because there is a size missing for the dimension of the ridge in the middle of the wheel. I have many spare tyres but see the inner wall ridge size are different widths and do not interchange. There need another size to be given on a spec doesn't it? Maybe like 20x10 plus x8 (as width of ridge)? So I can not find any tyres to buy even on the well known slot site for all the sizes have only two meansurements. There is Paul Gage tyres also except no choice for Scalextric and some other cars I have measured with caliper.