my warrior mini bike has 10:1 gearing with a 6.5 clone. 19" atv tires. and it gets up going well with a regular clutch. were you planning on using a centrifugal clutch, belt tensioner, CVT, or something else? 9:1 may be actually an okay ratio with a 6.5hp engine.
use the ratio calc on this website to determine your speed. remember the more speed it says, the less acceleration/towing power you will have. the lower speed, the higher acceleration/towing power.
when i get the CVT into my warrior, it will have a low ratio of about 13:1, and end up at around 4:1 i believe. thats alot of speed but still enough take off to pull you straight up any hill.
ill check that thread when you get it up.
good luck!