YOU THOUGHT $4 a gallon gas was a nightmare? How about $8? That's what you could wind up paying if you get hit with the dreaded "refueling charge" next time you rent a car.

Avis charges $8 a gallon to refuel.)

It's a fee that's easily avoided. Drivers have two alternative fuel options: You can fill up the car before returning it or prepay for a tank at the time of rental to avoid having to do it yourself. With the prepay option, the renter buys a full tank of gas at a fixed per-gallon rate, slightly below market price. For example, Hertz's new refueling policy, as of July 1, offers the option to prepurchase a tank of gas at 15 cents less than the market price in that particular location. (If gas costs $4.35 a gallon near Miami's airport, renters can prebuy a full tank at $4.20 a gallon.) Granted, the prepay option isn't a great deal if you're not planning to use a full tank of gas.

If you choose to refuel the car yourself but then don't, expect to get hit with inflated gas prices of between $5 and $8 a gallon, says Anne Banas, executive editor of Smarter Travel Media.

Source: Companies' web sites and spokespeople.

Beware the Unwanted Upgrade

Since smaller, fuel-efficient cars are in higher demand these days, renters should be more concerned about getting upgraded to a full-size SUV than downgraded to the compact.

"The chance of getting upgraded is strong. Sometimes you get an SUV instead of an economy car, and the rental might cost more in the end," says Banas. If you get stuck with a gas hog, ask the rental agency to pay for a tank of gas to help offset the cost of having to fill an SUV. "You can still negotiate," she says.

A Hybrid Is Not the Answer

When it comes to fuel costs, drivers going long distances might think renting a hybrid is more cost efficient. But in fact, choosing an economy-class car over a hybrid is often the cheaper way to go when you take per-day rates into consideration, according to Smarter Travel research.

Say you rent a Toyota Prius from Hertz at the Los Angeles airport on July 29. Your estimated rental bill will come out to $325 for two days (including tax, airport concession fee and facility charge), according to a search on Hertz's web site. Driving from L.A. to San Francisco round-trip will cost you at least $60 in gas, according to AAA's fuel cost calculator. Renting a Hyundai Accent will cost $247 for the same number of days, plus about $80 to drive the same distance. With gas costs, that comes to about $385 for the hybrid, compared with $327 for the Hyundai.

"If you're really environmentally-conscious, you might want to pay that premium. But if you're more concerned about cost, don't get the Prius," says Banas.

Also See:

• Worksheet: Should You Trade in Your Gas Guzzler?

• 9 Ways to Save on Gas This Summer

• 13 Simple Ways to Lower Your Electric Bill