It is my impression, that you want to turn the propellers at 9000rpm or less.
Typical rc car motors run around 18000 rpm and up, Hence the gear reduction on most motors.

Pat Matthews(patmat) says, " The propeller should be equal to the size of the motor can..."


Seems that if you can meet those requirements you should do alright.
If you make the propeller much bigger, then three or even five to one gear ration is not unheard of.