At least $8000
Wow, that is much less than I imagined. I thought we were looking somewhere at $50k. (10 drivers, each one with a school bus etc..)
In any case, there are Amex and Chase cards that offer 2x points for gas. 2 Amex or Chase points are worth around 3 cents. So apparently, even using the Ink cards straight at the gas station is a better cash back option than the mobil 2% card, unless you are going to hit 10k per month, in which case, the Mobile card might be better.