I am using baelfish to understand you but i think you want to know which motorbike to use on the different tracks. Here´s my selection.
Dirt arena offroad-7
Fun fair Street-7
Great wall Offroad-7
Lost ruins Offroad-7
Red city Street-2
Rock forest Street-4
Sea of sand Offroad-7
Snow ride Offroad-7
Speed bay Street-7
West way Street-7
Basically bike-7 on most tracks beacause off it´s speed. Remember to use manual gears too. You get a higher topspeed using manual.
Cheers,
Hooded.