Your math wasn't off. You didn't realize sales tax is not only charged on the gas but also on the 10% airport fee applicable to the gas.
I decided to accept the implied challenge and crunch the numbers. I'm assuming a 13 gallon tank. Gas rate is 3.56 and the "penalty" rate is 5.40. I'm assuming a customer does a minimal amount of shopping and is able to gas up for the same price of 3.56. A customer who shops should be able to beat the price by at least .05 /gallon. These numbers reflect the 10% airport fee and sales tax. In understand some people are willing to pay for convenience:
- Run out of gas while on the return line. The pre-paid gas plan cost you $8 more then if you had gassed it yourself.
- Return the car with 2 1/2 gallons. I'm assuming you won't drive more then 10 miles with the low fuel light on. The pre-paid plan cost you $17 more then if you gassed it up yourself.
- Pay the "penalty rate" and return the car with 3/8 tank of gas left. The pre-paid plan cost you $3 more then if you paid the penalty rate for 5/8 a tank of gas.. You can return the car with less then half of tank and still cost you less then paying for an entire tank of gas at the discounted price.
Conclusion. People who want to save money should never buy the pre-paid gas plan and should return the car full. People who want convenience should consider gassing up when they have less then half a tank and pay the penalty rate when they return the car. Running ahead of schedule. You have the option of stopping near the airport and filling up. Your choice.
Assume I'm flying out of Tampa. I go to DTD my last night. The low fuel light goes on as I leave my resort. I should have about 3 gallons of gas. I'll need enough gas to get to DTD, back to my resort and then drive to Tampa the next morning. What's more convenient?
- Give it a shot? Leave the interstate and try to find a gas station if it looks like I need it. Do you feel lucky.
- Stop at Hess by DTD and buy a couple of gallons of gas?
- Fill the car at Hess and pay the penalty rate when I return the car?
JMO but buying a plan which necessitates returning the car as close to empty as possible, but without running out of gas, isn't very convenient. Stopping even once to put a few dollars of gas in the car, so I won't run out, isn't my idea of convenient.
edit to add Declining the pre-paid plan gives you the best of both. You have the option of saving $$ or paying the rental agency for the gas you use.