"There's the psychological comfort of knowing you're driving a newer vehicle," MacPherson says.

With gas prices climbing past $3 per gallon, you might also be able to save significantly on fuel costs by renting a more efficient car if the one in your garage is a gas hog.

You'll have to find out if the desired rental car is available, too. Paula Rivera, manager of public affairs for Hertz, says the company's Toyota Prius fleet -- the hybrid sought after for its thrifty gas consumption -- is concentrated on the West Coast, where the demand is.

Consider putting a value on other features available on rental cars that your own doesn't offer. For instance, a DVD player might be priceless on a long trip with the kids.

Then, find out if you can swing any rental deals that sweeten that choice. AAA members, for instance, are eligible for a standard 15-percent to 20-percent discount from Hertz, which also offers discounts to AARP members, says Rivera.

On the other hand, the rental company may have provisions that increase the cost. Additional drivers can push up the bill. Teenagers, with whom you may have been counting on to share driving, may be barred from driving at all, MacPherson says.

He also advises confirming your own auto insurance will cover a rental vehicle. It's commonly believed that personal coverage extends to rental vehicles, but MacPherson says he's seen some policies that don't.

"If they do not, that could leave you very much exposed," he says. Drivers who discover their policies don't cover a rental could ask the insurer if a rider can be added for the term of the vacation, rather than buying pricey rental coverage.

Choosing the optional loss-damage waiver and personal accident insurance on the rental of that standard-size car from Avis, for example, would tack another $94.50 onto the bill, upping the total cost by almost one-third. A driver between the ages of 21 and 24 would also be charged an extra $25 per day fee.

Finally, see if your credit card will cover the deductible if an accident or damage occurs. MacPherson had this pay off when his rental vehicle was scraped by another car. He had to upgrade his credit card for the coverage, but that was free for the asking and saved him the $200 deductible.

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Cari Noga is a freelance writer based in Michigan