LOL That's what I did. It's too friggin hard to hit such small targets with your gun while you're driving at high speeds. My only problem was once when I backed up over a squad of Geth and accidentally drove off of the highway, falling to my death.
I loved doing that, but for some reason I was scared of the armatures and turrets... so after ramming some geth i didn't look bakc and just blasted on ahead. but then I found out the cannon-bullets strategy, shoot the cannon, and destroy what's left with the bullets. which as a quick way to destroy turrets. takes way longer (for me) to kill armatures, which is why I blast through them.
Which, is actually somewhat ironic since Tali and Garrus always happen to be with me on missions that have the Mako. What's even MOAR ironic is that the only time they weren't in the Mako was during the Noveria mission :3
Ashley's expression in the last panel made me laugh. And really, that's what I usually do when a game like Mass Effect has a vehicle section. Save on ammo, run them over. Or peck them off with your super zooming in over-kill cannon.