Without doing the whole experiment. I'd start with what's needed to get 100km/hr. 4000kv @ 8.4v should net 33600 rpm. Get the circumference of the tire (i'd advocate 1/10 for weight) then calculate what rpm the tire will need to spin to get 100km per hr. add some fudge factor but the difference between the rpm of the tire and the rpm of the motor will give you the ideal drive ratio. how you get there with pinions and spurs will be up to you. I'd look at a spur attached to the rear wheel in a direct drive type setup. Weight will be key to avoid cooking either the ESC or the motor but consider your aero so the thing doesn't get airborne.
Should be fun but I'd start with the math/physics before settling on the final solution. I doubt you learn the 6 Ps at Uni.... Proper planning prevents piss poor performance.....