I just changed it, so there is no confusion.
What is a bit strange, is that we don't find 17" or 16.5" rim diameter. So I wonder if the model is not :
Radius
<-------->
><===== : 1/2 Tyre
^^
Torus
With this "model", we have:
500 soft front : (28.75-7)*2/2.54=17.12"
500 soft rear : (30.5-11)*2/2.54=15.35"
1000 soft front : (29.5-7)*2/2.54=17.72"
1000 soft rear : (32.5-13)*2/2.54=15.35"
The other possibility is that we don't care about the rim diameter, but only about the tyre diameter.
Edit: I just had a look to the Scooter tyres, and to have the 10" wheel size, they are also based on this model (Tyre Radius - Torus Radius/2)*2/2.54 = 10"
What is a bit strange, is that we don't find 17" or 16.5" rim diameter. So I wonder if the model is not :
Radius
<-------->
><===== : 1/2 Tyre
^^
Torus
With this "model", we have:
500 soft front : (28.75-7)*2/2.54=17.12"
500 soft rear : (30.5-11)*2/2.54=15.35"
1000 soft front : (29.5-7)*2/2.54=17.72"
1000 soft rear : (32.5-13)*2/2.54=15.35"
The other possibility is that we don't care about the rim diameter, but only about the tyre diameter.
Edit: I just had a look to the Scooter tyres, and to have the 10" wheel size, they are also based on this model (Tyre Radius - Torus Radius/2)*2/2.54 = 10"