If you're driving around with the Mako on any planetary surface, whether on a side quest or during the game's main missions, you can use the Mako's machinegun and missile launcher to do massive damage to enemies. However, the extra power of the weapons means easier kills, resulting in lower experience points. If you want to get more experience (twice as much, in fact) get out of the Mako to kill enemies. You can weaken them as much as you want with the Mako, run them over, pin them down, and more. But be sure to deliver the killing blow outside of the Mako to maximize how much experience you will earn from the kill.