A common driving nightmare is getting a flat tire, especially on a busy highway or remote byway.

Used to be that almost every vehicle carried its own fix: a full-size spare or a smaller so-called "doughnut."

No more.

About one of every eight new vehicles sold in the first half of last year had no spare of any kind, according to AAA Midatlantic.

Although more than a handful of models - including Chevrolet Corvettes, Toyota Siennas, Dodge Vipers and Porsche Boxsters, according to one list - dropped spares at least five years ago, the trend has been growing

Instead of spares, cars come with a can of sealant and an air-compressor, or rely on so-called "run-flat" tires.

The aim is to improve gas mileage, especially with tightening federal fuel-efficiency standards, and it does make a difference, AAA acknowledges.

Losing 40 pounds of tire and jack can lower fuel comsumption enough to give an extra mile per gallon, saving nearly $1,000 over the course of 150,000 miles, the roadside assistance group calculates.

But the no-spare approach has drawbacks, experts warn.

AAA Mid-Atlantic handled more than 300,000 tire-related problems last year, and in 1,223 cases the driver lacked a usable spare, necessitating a tow.

Sealants can fail if there's sidewall damage, warns Consumer Reports.

Sealants also lose effectiveness over time, and should be replaced every five years, AAA said.

"Run-flats can sound convenient, but they can be costly and prove difficult to find replacements," the consumer publication writes.

New-car buyers are advised to ask about spares, and if they're not included, find out if one can be added an option.

"For those who travel to remote locations, where help could not be readily summoned, it may be worth the investment to buy a spare tire," Consumer Reports suggests.

"Unfortunately many vehicle owners may be unaware that their vehicle has no spare tire until they experience a flat tire. It's a safety issue and we would prefer to see vehicles equipped with a spare," said Jenny Robinson, manager of public and government affairs for AAA Mid-Atlantic, which serves 4 million members in Pennsylvania, New Jersey, Delaware, Maryland, Virginia, and Washington, D.C.

Drivers who do have spares should regularly check them to make sure they're still properly inflated, AAA suggests.

Taking the time to familiarize a family's drivers with how to change a flat "will make it much easier to address a roadside emergency, especially under less-ideal conditions, such as foul weather or alongside heavy traffic," Consumer Reports notes. "In doing so, you may identify ways to better prepare, such as supplementing the included lug-nut wrench with a longer torque wrench that may be easier to use."

Or buying a sturdier jack.

Contact staff writer Peter Mucha at 215-854-4342 or pmucha@phillynews.com.