In 2007, the average selling price stood at $28,797 for new cars and $15,715 for used cars in the United States. When a car payment is added to the cost of insurance, maintenance, repairs and fuel, operating a vehicle is no small expense. As a result, many people settle on a vehicle and keep it for years. Some 60 percent of vehicles on the road in 2001 were older than seven years. The average age of vehicles in operation in 2007 was nine years. That said, older vehicles can be money pits.  You could pay hundreds of dollars or even $1,000 for a major repair.

How do I know when to replace or repair an aging vehicle?

Sometimes, an old car that is paid off, reliable, familiar and need not be protected against every potential scratch or spill may be worth keeping. But a car that frequently breaks down is a serious candidate for replacement because of the cost of repairs and the aggravation factor. Unfortunately, there is no easy formula. Replacing a car is a personal financial decision and often a compromise between what you want and can afford.

Bryan Gregory, director of consumer education at Advance Auto Parts, said one approach is to compare what you are spending on maintenance with what you could spend on repairs to keep an older car in running order. He estimates that maintenance for one vehicle will cost about $700 a year. That's for oil changes and air filters needed periodically and for the proportional cost of new tires and brakes not necessarily needed each year. In his view, it's time to consider replacing a vehicle when the annual cost of repairs -- that is, hard parts such as the installation of a new starter -- approaches $700 also.

This analysis assumes that a consumer is recording each dollar invested in the vehicle. Gregory does that. He knows not everyone does. Without such records, another approach that comes from AAA, the automobile association, might deserve a look.

It suggests that, after receiving an estimate for a major repair, you compare it with the vehicle's value. "When the cost of a needed repair approaches 50 percent of the vehicle's value, it is time to seriously consider replacing it," a news release said. By "value," the organization means what the consumer would receive for the vehicle in a trade or sale.

How do I know what my vehicle is worth?

Take into account the make, model, year, condition and mileage. High mileage reduces the value of a vehicle. Excellent condition improves the value. Various Web sites, such as www.edmunds.com, can help.

Does a large repair ever make sense for an old vehicle?

Yes, said Jack Gillis, a longtime auto industry observer who works at the Consumer Federation of America. A consumer who experiences a catastrophic mechanical failure such as a blown transmission may quickly conclude it's time for a new vehicle but, Gillis said, look again. He once paid $5,000 for a replacement engine for an old, paid-off Volvo wagon worth about $7,000, grossly exceeding AAA's 50 percent rule.

But Gillis said he drove the Volvo for an additional 50,000 miles. The cost of the repair spread over the time he drove those 50,000 miles turned out to be much less than what he would have had to pay to buy and operate a new or newer vehicle during the same period, he said.

"The break point is not so much a hard line in the sand, but what is it going to cost you to replace the car?" Gillis said.

In Gillis' estimation, an owner should compare the cost of a repair that will keep an older vehicle in running order for at least two years, and ideally longer, to the total cost of replacing the vehicle for the same period. The owner's replacement cost will be the sum of the monthly payment for the purchased vehicle (or monthly payment increase if the current vehicle is not paid off) and any increase in insurance.