Driving with either a) air conditioning on; or, b) windows rolled down

I drove to Phoenix in the summer with this girl who just would not put on the A/C because she wanted to save gas (not an environmentalist, just cheap. though she threw away money on frivilous things, so maybe she was just a mentalist). I told her driving at high speeds with the windows down was worse for gas mileage---I had no idea if it was or not, but goddamn it was hot up in that bitch---and she basically just said 'oh that's nice. doot da-doo duh-doo' while continuing to never put on the A/C. Even when we got to Phoenix and it was 110°. I bet if I had just killed her, a sympathetic jury would've let me off.