mmm combo on rc8

in about 2 weeks i get all the stuff i need to finish my rc8 conversion.

im well be using the following

rc8
ae conversion
6s 4000mah 25c
mmm/2200kv combo

i dont know what i should do about gearing. i dont want it to over heat but i want it to have amazing amounts of power. like kill my nitro buddys. i think there cars are going about maybe 45mph.

also what kinda run times do you think i should be getting with the 4000mah 6s battery packs. im hoping for like 15 to 20mins.