1/24th scale PU Tyres.
 
These tyres are sold as a pair and can be streched over rims from 18mm upwards. They do not have a rib groove in them and can be cut down to size in depth. Also being PU rubber, they can be sanded down in diameter, so for example, you can sand the fronts down to a 3mm wall from the 4mm to give the look of a smaller front tyre. 
 
I have two Youtube videos showing the use of these tyres for both front and rear tyres and how they can be reduced in size. Please see links to the right of these words.
 
PU Rubber
26mm OD
18mm ID
20mm deep
4mm wall