Lower KV big motor with lots of voltage will get you the most speed.

Look at a 9000kv motor, it's a 540 can limited to 2S. So you're talking (in a VERY simplistic way) 9000rpms per volt with 8.4volts so 75600rpms (no load, frictionless) The problem with this design is there's VERY little torque, both due to the KV (the higher the KV the higher the speed but the less torque, just like turns in an old brushed motor) and due to the fact that it's a small motor (comparatively speaking) so you lose a lot of that power because you're under load with friction.

Alternatively, if you take a Neu-Castle 2200KV 1515 motor on 6S (25.2 volts) you achieve 55,440 rpms under no load or friction. The difference is because you have a lot more volts (power) running through the system and a MUCH bigger motor, you have a HUGE amount more torque to put those RPMS into play.

So what happens is when you run the 9000KV set up, you have friction and load which creates resistance against moving the vehicle. Because the motor isn't very strong (just high revving) and there's not a lot of power running through it, it is more affected by that resistance (which shows up in heat) and can't get any where near the "no load RPMS". Where the 2200 1/8th scale motor on 6S isn't as affected by the resistance because it has the TORQUE to overcome it.

Look at full size vehicles, when you're building a dragster or race car, do they use little 4 cylinder engines...no. They use much bigger motors because you want both torque AND horsepower.

If you want pointers, just look at the Traxxas XO-1. Look at the system in that for it to reach 100mph.

EDIT: I forget, what kind of car are you putting this in? If it's a 1/10th pan car...then the 9000KV is your better option because there's almost no load to that car, if you're talking a 4wd short course lets say, or a 1/8th buggy...than the big motor high voltage is the way to go.