Quoting jacob rees
Where did you get those tyres?
Here are the Peeron/Bricklink references : rear wheel = 1 wheel 6014b (normaly used with tire 6015) + 2 tyres 50945 (normaly used with wheel 50944px1); front wheel = wheel 4624 + tire 3641 that I made smaller with a stationary belt sander (7b tire can be used too but is bigger)