It may sound expensive to rent a car instead of driving your own.

But Brett Smith of the Center for Automotive Research in Ann Arbor, Mich., says that sometimes it's cheaper.

The reason? That "free" trip we take in our own car doesn't take into account the depreciation of your vehicle when you rack up the miles.

"Granted, you will pay more out of pocket to rent a vehicle in the short term. But longer term, it's a much different equation," he says.

In a recent project he did for Enterprise Rent-a-Car, Smith calculated the cost of driving 300 miles over a weekend in a 5-year-old driver-owned vehicle compared to driving a full-size vehicle rental at $19.99 a day or a compact rental at $9.99 a day. Although the full-size vehicle rental came out just a penny less per mile than driving your own car (63 cents per mile), the compact, even including the rental cost, was 12 cents per mile cheaper.

And if you drove more miles, it would be an even greater savings. "If your per-mile costs are cheaper and you can spread the rental fee over more miles, it becomes even better," says Smith, who is co-director of manufacturing, engineering and technology at the Automotive Research Center.

The study assumed gasoline cost $4.15 per gallon, on the high end of what may be expected this summer.

Although the research was done for Enterprise, the calculations Smith used could be used by anyone to figure out whether it's worth choosing a rental car for a particular trip.

He calculated that it costs 63 cents per mile to operate your own car -- in gas, oil, maintenance, tires, insurance, registrations and depreciation.

A rental car costs just 39 cents per mile to actually operate, because although you still have to pay for gasoline and a few other things, you are not spending money in depreciation, tires, maintenance or oil.

Of course, a car rental also must include the per-mile cost of renting the car (total rental cost divided by how many miles you are driving). Smith calculated the rental alone cost 12 cents a mile for the compact car and 23 cents for a full-size -- making the total cost per mile of a compact rental car 51 cents per mile and a full size 62 cents per mile.

Although the study assumed that the car you own is 5 years old getting 24 miles per gallon, a person who owns an older car could realize greater savings, Smith says.

Bottom line, travelers should do the math, Smith said. "It's a way to explore the costs we think are free versus the real costs."