Wondering whether to rent a car or drive your own for that summer road trip? Pull out a calculator. It all comes down to math.

Determining whether it's more economical to rent a vehicle or drive your own boils down to three things:

Let's start with driving costs. AAA's 2006 vacation planning guide suggests budgeting $17.10 per 100 miles. That includes gas, figured at $2.92 per gallon, tires and maintenance, and assumes the vehicle gets a rather efficient 25.8 miles per gallon.

By the numbers ... driving your own vs. rental

Driving your own car Budget per 100 miles (gas and maintenance) $17.10 Multiply by number of miles x 1,000 Total baseline cost   $171.00   Driving a rental car Weekly rental rate (standard-size car) $222.59 Plus cost of gas ($2.92 x 1,000 miles) + $113.00 Total baseline cost $335.59

So say you've got a 1,000-mile, week-long trip planned. Multiply 10 times $17.10 for a baseline cost of $171 to drive your own vehicle. It'll be less expensive for smaller, fuel-efficient cars and more expensive for bigger gas guzzlers.

Now the rental scenario: Avis quoted a rate of $222.59 for a weekly rental ($209.99 base rate plus $12.60 tax) of a standard-size car in suburban Chicago in mid-July. Mileage was unlimited. Averaging the 25.8 miles per gallon AAA figures in, you'll need 38.7 gallons of gas for your 1,000 mile trip. At $2.92 per gallon, your gas tab is $113, bringing the rental cost to $335.59, or almost twice the cost of driving your own car.

For a weekend trip, however, the costs come much closer. Avis quotes a rate of $47.98 for that same car for a Friday to Sunday rental plus $2.88 tax. Gas for a 500-mile trip -- calculated at the miles and cost per gallon used above -- will add another $56.58 and bring the bill to $107.44. Meanwhile, driving your own vehicle on that trip using AAA's estimate will cost $85.50.

After you do the math, consider other factors. Drivers who lease their vehicles should find out whether they're approaching the mileage limit. If the vacation will put you over the top, it might be more economical to rent.

"Each lease is different. You have to know what the per-mile charge is if you go over your miles," says Jim Rink, public relations consultant for AAA Michigan.

Jim MacPherson, auto columnist for The Hartford Courant, says lease holders should also be sure their destination is permitted in the lease agreement. Mexico is typically precluded and Canada may be as well, he says.

Rental cars also save wear and tear on vehicles and can provide some peace of mind if you're not sure your car is reliable. A breakdown can spoil a vacation as certainly as a missed flight. Moreover, you'll then be forced to rent from whatever choices are nearby, rather than doing a competitive search prior to departure.