I found this on an old forum (2008). It will give you a starting point.
4.00x12 is a pretty high profile tire! That's equivalent to a 100/100-12.
Most of the 12 inch wheels I see are using 120/70 or 130/60 tires, which have about the same outside diameter as 3.50x10 tires. (The manufacturers do that so they can advertise "larger" 12 inch wheels while still using standard GY6 engines and drive trains!)
Good luck on your quest to find a replacement tire!
Read more:
http://scootdawg.proboards.com/threa...#ixzz3iEvXRVgr