Before you need to reach for a spare, make sure one’s there, warns AAA Carolinas.

One in every seven cars sold today comes without a spare tire. More car manufacturers are dropping spares to make cars lighter and boost gas mileage. These cars include a tire repair kit and sealant instead, but they don’t work on every type of flat tire, according to AAA.

At a demonstration Thursday, the automobile association opened the trunk of a Hyundai Elantra, which had an unfilled cutout where the spare tire typically would be. A repair kit was in there instead.

“The public doesn’t really know about this – they discover this when the tire goes flat,” said Tom Crosby, spokesman for AAA Carolinas. “(It’s) one of those subtle things creeping into the market.”

The local division of the automobile association, which provides roadside assistance and car maintenance, started noticing this trend recently and wanted to raise public awareness, representatives said.

Getting rid of the spare tire is a bragging point for some manufacturers.

“Changing a flat tire on the side of a busy freeway can be a dirty and dangerous job,” states a Chevrolet news release.

“The 2011 Chevrolet Cruze offers a safer option: an on-board tire inflator kit that also helps improve fuel efficiency by shedding 26 pounds of spare tire and jacking hardware.”

Lighter cars help automakers meet federal fuel economy standards – now at 29.7 miles per gallon, and increasing to 35.5 by 2016, according to AAA.

“I think you will see more cars on the market that have no spares at all, but I think they will be in segments where people do not expect to use it as a long-distance highway car,” Paul Williamsen, national manager of a training program for Lexus dealers and their staff, told The Detroit News last year.

AAA says removing the spare brings only a limited boost to mileage. And using sealant brings its own problems.

If the tire puncture is too big, the sticking agent drips out, and doesn’t take. Its gooey consistency can clog up tire pressure sensors. And tires that can’t be cleaned out to fix the puncture need to be replaced, according to the association.

AAA Carolinas offers these tips:

• If you are buying a new car, check for a spare tire. Buyers aren’t always told the extra tire doesn’t exist, according to Dusty Holcomb, president of AAA Carolinas’ Car Care Centers. Average cost to add a spare is $150, Holcomb said.

• If you have a new or nearly new vehicle, check to see what’s in the trunk. Some models started swapping out spares for tire repair kits starting in 2006.

• If you have a tire repair kit, review the instructions. Some come with an air compressor that plugs into the car’s charger to speed up tire inflation.

• If you use sealant, limit the tire use to 50 miles, or the adhesive hardens up too much to remove.

• Check your spare tire at least annually to make sure it’s inflated.

• Tire repair kits should be replaced every five years or after usage.