To gain extra experience from battles, make sure to get out of the Mako for the final hit. You can weaken the stronger enemies from inside the Mako and then get out to finish them off. You will only get 60% of the experience if you kill enemies with the Mako.

When you are using the Mako, try to kill as few enemies as possible with it. You can even use the Mako sitting in the garage on Noveria that you can decrypt and use (only the guns), because it is worth the same as far as experience points earned. Most of the time you will gain more experience points if you defeat enemies with your own weapons. In the case of a Thresher Maw, Heavy Turret, or a Geth Colossus, just use the Mako's cannon and mini-gun to get them to a sliver of health, then finish them off with your weapons. You will gain the same amount of experience points as you would have if you had fought them entirely with your own weapons. The sniper rifle can also earn you more experience points if your character is trained with it. Be careful on planets with hazardous environments when using either of these methods.