It's never a good idea to return a rental car with less than a full tank of gas. The reason? The rental car companies usually charge a fee if you don't fill up before dropping off. And according to a report in USA Today, that fee has, in some cases, gotten a lot steeper.

The national newspaper polled 102 rental companies at 13 major airports to gauge how much is being charged per gallon for renters who don't fill up before returning their loaner. Surprisingly, 54 of those airports were charging less than the average fuel price at local stations, likely because many companies haven't adjusted their per-gallon charge to reflect rising gas prices. Another 43 rental stores were charging more than the average gas station.

Meanwhile, the Hertz rental company is in a league of its own. The USA Today poll shows that the rental giant was charging $9.29 per gallon of petrol at 13 locations. Dollar and Thrifty weren't far behind at a pair of locations, with a price of $8.99/gal. For perspective, at $9/gal, it would take around $275 to fill up the typical full-size pickup, and about $110 to top off a Ford Focus.

While those are some ridiculous pump prices, this is one instance when the punishment is nearly 100 percent avoidable. After all, if you don't want to get slapped with a $9.29/gal bill, all you need to do is fill up before dropping off the vehicle. Unfortunately, you're still going to pay well over $4/gal in most areas.