Depending on gas prices in your area you need to spend less or more. If price per gallon is $3, 5c promo requires $6k in GC spending to get up to 20gal for free in single fill up. ($3/.05) x $100 = $6k. 15c promo requires $2k. ($3/.15) x $100 = $2k. Average tank capacity is 14-17 gal and most of the time is not completely empty. So for example if 12 gal x $3 = $36 and you spend $2000 + $23.80 fees + $1.76 MO fees and earn $40.28 cash back on citi DCB the profit that includes costs of gas is $50. 20 gallons fill up translates to $74.72 profit and If you have to spend $6k because of 5c promo the profit is $106.80.