With rental car companies always on the hunt for new ways to generate cash, it's no surprise that the gas tank is viewed as a potential profit center. Convenience charges (if you don't want to refuel yourself), penalties (if you didn't sign up for the convenience charge beforehand and forget to refuel); even fees for bringing the tank back on Full – but not full enough... Hey, we understand – it can all get a little confusing.

The first thing to remember as a renter: They want you to be overwhelmed. Confused people will spend more money. Don't let the guys behind the counter smooth talk you into forking over any unnecessary cash; check out the handy AutoSlash guide to your refueling options.

Option #1 – Return with a full tank

This one's easy. You promise to bring back the car with a full tank of gas (or, in many cases, with as much gas as there is in the tank when you leave – make sure they note on your rental agreement if it's less than full). You'll need to make sure that you leave enough time to fill up before heading back to the rental location—especially if you are trying to catch a flight.

It's always a good idea to note the gas stations closest to the rental location when you pick up the car, to ensure that you're not wasting time searching for a gas station on your return trip; however, also note how much they're charging per gallon, then weigh this against gas prices elsewhere in town.

Many gas stations near airport rental car return facilities charge above market rates, since many renters are in a hurry to catch their flight and don't have time to shop around. One gas station near the Orlando airport is infamous for overcharging by $2 or more per gallon! Check out this message thread on Flyertalk, a website geared toward frequent flyers, with more than 130 entries (going back to 2005) with people venting about this place.

Option #2 – Prepay your fuel costs

At first blush, this seems like a pretty good option. The rental car companies typically offer an attractive discount per gallon for pre-purchasing gas through them. You could be attracted to this “too good to refuse” offer, but wait a moment: When have car rental companies ever done anything that does not involve them making a profit?

The catch here is that you basically have to return the car sucking fumes in order to recoup your investment in that great deal you got on the tank of gas. The rental company will not refund you for any unused gas in the tank.

Plus, unless you know the fuel capacity of the gas tank in the model you are renting, you may – oh heck, let's face it, you are -- overpaying for the amount of gas actually in the car. We've yet to see a prepaid fuel charge on our final statement that comes anywhere close to the amount it actually cost us to fill up the tank at any point during the rental period. Additionally, don't let them sweet talk you into paying the fee with promises of lower prices than at any nearby gas stations. That can be true when they're referring strictly to airport-adjacent options (see Option #1), but not necessarily just beyond the airport. A good rule of thumb is to check gas prices in your destination before leaving home, easily done on a site like Gasbuddy.com.

Option #3 – Winging it (or, Hello, You're Screwed!) Running late?

Didn't have time to fill up? Boy, is that going to cost you.

While some states have taken measures to curb the amounts rental car companies are allowed to charge as a penalty in this case, it's still going to mean paying a per gallon price so exorbitant, one might be tempted to think that the next renter would end up driving around with rocket fuel in the tank.

We've seen charges of $6.75 per gallon. Ouch, right? This, in cities where the local average was just $2.85! If you're chronically late for everything, best you should consider Option #2.

TREND WARNING: NOT FULL ENOUGH!

While we've yet to experience this firsthand, many renters are complaining that companies are asking to see gas receipts when you return, to ensure that your tank is actually full – and that it doesn't just say Full. (Most cars can take a couple of gallons past the Full mark.) A good rule of thumb is to fill up within 10 miles of the airport. Though, like we said, receipt checks aren't all that widespread at this point.

Another complaint going around: If you didn't drive your rental more than 75 miles, some companies may ask to see a gas receipt to show that you put at least some fuel in before you returned. Budget, for example, has been known to charge an “EZFuel” fee of $13.99 as a penalty if you tried to get away with freeloading off the top of the tank.

Really want to play by the rules? (Nerd alert!) Stop off at the first gas station you see after leaving the rental lot; fuel up. Then call in and ask the agent to note that you filled up the supposedly Full tank, and that you have a receipt to prove it.

On return, fill up completely again – heck, do it at the same gas station – then show both receipts and ask to be credited for the first one. After all, the agreement says you had a full tank when you drove off the lot – a time-stamped receipt is all you need to prove them wrong.