June 10th, 2008, 1:53 pm
In order to get the 9% discount you must spend $150 in the previous month (month 0). Since you only use $100 monthly, no matter how you refill during the previous month, you will have at least $50 worth of gas left in the tank. Thus, during the current month (month 1), when you have a 9% discount, you can get at most $130 (= $50 + $80) before the discount, which means you'll save at most $11.7 and will have a full tank by the end of the month. To keep getting the 8% discount you have to spend at least $50 in the following month (month 2), which means you'll have about $30 worth of gas left by the month end. During month 3 get the minimum amount of gas, about $70. You can restart the strategy in the 4th month.Hence you got $400 worth of gas, $130 of which at a 9% discount, the rest at an 8% discount. So you saved $33.3 in 4 months.If you just put $100 every month you'll save $32 in 4 months, so as Demyan said, the $1.3 doesn't seem to justify the effort.
Gas station has a new promotion but Im not sure how to make best use of it
Every calendar month if I spend 50 dollar or more (but less than 150 dollar), next calendar month I can get 8% discount Every calendar month if I spend 150 dollar or more, next calendar month I can get 9% discount My average monthly consumption is about 100 dollar. Consumption is linear.So what will be the best re-filling strategy for me (assuming its first day of the month and I have an empty tank) ?Some constraints:My gas tank is only for 80 dollar worth of gasoline. (yes its a small car 
And I may not want to bring a big kettle to gas station but if it happens to be the best strategy please let me know)Its real life problem so Im more willing to see realistic solutions. (i.e. please avoid strategies such as adding 2 liters every day :-)thank you! 
