hi, i had this old weed wacker laying around so i decided im gonna put it on my bike and have a motorized bike. so everythings set, i have the bike, the engines rdy to be installed but i have a few questions. im gonna do a gear ratio of 18.75 like i hear most ppl but how do i do that? i kno its with sprockets and stuff and the amount of teeth on them but how do i get the proper gear ratio? i kno i sound stupid but i rly dont kno that much and wanna learn. plz help thanks.