gear ratio;s

[jimbob]

im thinking of using an outrunner brushless motor
it will have 12 poles :018:
im wondering what gear ratio's everyone thinks i will be able to get away with
it will only do upto 20,000 rpm
just wondering it i can get away with a gear ratio of around 7~9:1
or will this be easy for a outrunner with 12 poles
i see with the inrunners probably with 2-4 poles they run around 15:1

it will be running 6" tyres