Thanks for the idea, but it's diameter is 48 mm
And I think it's still looking big (You can see V8 engine inside, but it is not final version, i'm trying to make it more compact)
With that wheels I need make car longer again.. So as you can see from my calculation's table, If I want to build 1:15,5 scale, it should be 17 studs in width, but contest's limit is 15. It will be incorrect proportions.