you know what gear to go in because you dont want to be on full revvs in a low gear. or taking a long run up on a high gear
what ever feels sensible really
i go for gear 2 on most things or sumtims 3
because the air is so cold the air is very dense, so the engine takes in 2wise as much air as usuall. so the petrol air mixture needs to be ajusted on your carb.