Math Helps Resolve Whether to Sell Car : Autos: Decisive factor is new car payment. Keep old car if reliability is not an issue, says one expert.

It runs like a dream, starts in the dead of winter and does a comfortable 65 m.p.h. with nary a shake. But it’s 4 years old, doesn’t have a sun roof and, well, red is no longer your color.

When is it time to sell the car and plunk down many grand for a new model? Conversely, when should you hold on to a good thing?

Put aside, if you can, the subjective considerations of image, comfort and the desire for something new. That’s a whole different story. For most, buying a new car or keeping an old one is a decision based on cost.

If money is not an issue, drop this and head down to your local car dealer. If dollars do count, go get a pencil and paper.

Using a financial analysis developed by consultants at Runzheimer International, consumers can decide whether to sell or hold, based not only on the resale value of their car but on what the car will cost in real dollars over time.

Let’s say you own a 1991 six-cylinder, four-door sedan. The car you are considering buying is a 1995 six-cylinder, four-door sedan. Your car cost $14,493 and gets 21 miles to the gallon. The new car costs $17,359 and gets 21 m.p.g.

Your current car is paid for, and you’ll have to finance the new one with a four-year loan at 8.5% annual interest. The down payment for the new car is the trade-in value of the old one.

“This is not an unusual decision to make, whether to sell or hold, because it’s a depreciable asset that costs a lot of money,” said Lee Czarapata of Rochester, Wis.-based Runzheimer.

“The new car payments are the decisive factors,” he said.

“Even through the newer-model car has a much greater trade-in value after four years and you save on repairs and maintenance, the monthly car payments, which total slightly over $16,500 in principal and interest over four years, more than counterbalance the factors,” he said.

In Runzheimer’s analysis, the 1991 model’s total costs after four years--payments, fuel, licenses and repairs--would be about $12,308.

Four years from now, the new car will cost $27,361, including those expenses plus the cost of principal and interest on the car loan.

Over four years, the 1991 model requires $100 more for fuel and oil (totaling $3,580) and $2,231 more (a total of $4,449) for repairs, maintenance and tires.