C
Assume a fleet of 1000 cars driven an average of 150 km/week at 15 kpl. You'll pay for 10,000 L per week on the average, no matter what strategy you use. Every single week, you're either paying for 10,000 L for last week, or for this week, or for next week.
You realize you're describing the difference between annuities immediate and annuities due? The difference in their value is exactly the interest earned in that first period, multiplied by the annuity amount. If we're talking 40,000 L a month, that could be on the order of $2000 for negotiating an arrears payment plan instead of an advance one.
Also, consider the effect of excess capacity. Your gas filling station can handle, say, 30,000L (or 120,000, so it will last for 90 days). How much should you buy? Your total capacity is effectively a "real option". Put into the language of vanilla options, you should buy as much as you can when the "dividend" (i.e., the saving incurred by owning) exceeds the financing cost of buying.