Sometimes you meet someone who takes a rough idea and polishes it into an art form.

Meet Jim Williamson, an East Falls, Pa., pizza shop owner who has figured the best way to cut the costs of car ownership to the bare minimum.

In a recent column on this subject, I argued the best approach is to buy a good used car and drive it to death, then get another one.

I relied on data from Kelley Blue Book, the car-valuation folks, that said the average car loses 65 percent of its value in its first five years.

Cars depreciate, or lose value, very fast in their first few years, but at a much slower pace in the latter ones.

So the idea is to let someone else take the big depreciation hit, then buy a car at a third of its original cost — while it still has two-thirds or three quarters of its useful life ahead of it.

Williamson has refined the idea.

He buys a 5-year-old car. But instead of keeping it until it dies, he sells it two or three years later — often getting almost as much as he paid, while avoiding the higher maintenance costs of a car on its last legs.

“Very simply, there is a window of three years, from (age) five to eight years, when a good used car does not depreciate very much and mileage, from 35,000 to 70,000, only makes a negligible difference in its selling price,” he says.

In the classifieds, for example, a seller will typically ask for about $7,000 for a 1998 Honda Civic EX with under 50,000 miles, he says. By showing up with cash or cashier’s check, you should be able to get the car for $6,200, he says. Three years later, you should be able to sell the car for $5,500. Total loss for three years: $700.

That’s less than a couple months’ payments on a new car.

Nothing’s guaranteed, of course. Using the Kelley Blue Book site, www.kbb.com, I came up with a purchase price $7,320 for the same car and a sales price of $4,565 for one three years older, for a difference of $2,755.

That’s still pretty cheap for three years’ use of a sound vehicle. And the Kelley figures are just averages. By holding out until you find a seller who’s eager to unload, and then avoiding being that kind of seller yourself, you surely could come closer to Williamson’s figures — or beat them.

In fact, he suggests you keep the car-hunting going most of the time. This is how he got his current vehicle, a 1998 Toyota Avalon.

“Look for a friend, co-worker or neighbor that buys a new car every five years or so — someone who drives a good quality car that you like, someone that takes good care of their car, someone that usually trades in rather than sells outright,” he says.

“Offer $500 more than what the dealership would pay them for a trade-in. This amount is almost always less — usually $1,500 less — than what they could sell it for in the newspaper.”

Williamson says he paid $350 a month for his first car, a new one, 15 years ago, before adopting his used-car system. By instead investing that amount in a mutual fund, he’s amassed more than $80,000, he says.

“I have used this money to pay off my mortgage, and I’ve used the mortgage savings to grow this account even more,” he says. “I could go out tomorrow and buy the biggest, baddest Mercedes my neighborhood has ever seen, but I’d rather have the most impressive portfolio on my block.

“House paid, kids through college — no problem because I drive a 5- to 8-year-old car and invest the savings.”

Lots of people are understandably leery of used cars. Who knows how an older car has been treated?

If you’re not a mechanic, have one you trust check out any car you’re considering buying.

The American Automobile Assn. offers a list of approved service centers, and it sells extended warranties for used cars. Three years’ coverage for that Honda would run about $900. Check the Web site, www.aaa.com, or call (800) 323-4300.