You do need to know the diff ratios, though if you told us what vehicle you had and any other useful info like tire diameter and intended use (with a motor kv that high and 4s lipo one assumes speed runs?), it would make life easier- if its a common vehicle, all the required info will be there in the calculator, just choose/input the applicable info and it'll spit out the estimated speed with any given gearing- go up or down on the spur and pinion until you get the desired result
