It was once a huge red flag: When a car’s odometer would hit 100,000 miles, “it was almost a magic threshold that meant the car was probably worn out,” says Kay Wynter, who runs an auto service center in Fort Myers, Fla., with her husband, Terry.

But thanks to improvements in car design and maintenance, the milestone of 100,000 miles now means something very different.

Although some cars are ready for trade-in at that threshold, many others can travel twice as far without major repairs.

What allows one car to pass the 100,000-mile barrier with few repair bills, while another is ready for the junkyard? It’s all about preventive medicine.

“It’s just like when you get to be 70 and everyone tells you the same thing: Exercise, eat right, take care of yourself,” says Lauren Fix, author of “Lauren Fix’s Guide to Loving Your Car” (St. Martin’s Griffin, 2008).

Feeding your car the right things and taking it for regular checkups will make all the difference.

Open the book

The key to keeping your car running smoothly is probably tucked at the bottom of your glove compartment, under the spare napkins and ketchup packets. It’s the owner’s manual, which most people ignore at their peril.

“There is a schedule in the manual that runs well over 100,000 miles,” says Fix, and it lists when to replace parts likely to be wearing out. The list will vary for different cars, so check yours and follow it.

Newer cars may have the maintenance schedule built into an internal computer. A blinking light or a beep will announce that it’s time to replace certain parts, says autoeducation.com founder Kevin Schappell.

Get fluent about fluids

The liquids that go into your car (gas, oil, brake fluid, power steering fluid, etc.) are crucial to its survival. To extend the life of your car beyond 100,000 miles, these experts suggest frequent oil changes and fluid checks done at dealerships or full-service auto centers.

In choosing oil, Fix advises buying full synthetics. They “actually will lube the engine better. It’s designed for longer life. There are less emissions, so it’s greener. There’s slightly better fuel economy and better performance,” she says. “There are no negatives except it costs a little more.”

Gas also matters: Different cars benefit from different types, so check your manual.

Find the right shop

Sticking with your car’s dealer can be a safe choice, because the staff will be trained to work on your car. But over the life of a high-mileage car, regular maintenance at a dealership can get pricey.

“Rates at an independent shop may be about $40 to $50 an hour,” Schappell says, “but you’re paying probably $60 to $90 an hour at a dealer.”

The cost of repairs can vary widely depending on the brand of car. Parts for some vehicles, including exotic cars and some German models, can be hard to get, driving up their cost. That can be a reason to trade in a car just before the 100,000-mile threshold.

When you do replace parts, there are ways to save money: “A quick oil-change place will charge you $50 for an $18 air filter,” she says, because you’re mainly paying for labor.

But an auto-parts store will charge you only the $18 price tag, she says, and “you can buy it and say, I don’t know how to put this on. They’ll do it as a courtesy.”

Type of miles matter

Highway driving puts less stress on a car that tooling around locally. It requires less quick braking and acceleration, and moisture under the hood has a chance to evaporate.

“Cars that do a lot of short trips will require exhaust work a lot sooner than car that travels on the highway a lot,” Schappell says.

Local driving in colder climates can also cause buildup of ice and snow under the car, which may contain corrosive chemicals. Fix suggests hosing it off on slightly warmer days. She also suggests waxing your car regularly.

Sound like a lot of work to keep a car zooming along past 100,000 miles?

“It’s your second most expensive investment. You want to take care of it,” says Fix.

“With your home, something needs fixing and you get on it,” she says. “With your car, especially one with a lot of miles you have to get on it right away, too.”