If you’re driving a car made in the last five years and you plan on road tripping home for Thanksgiving this year, make sure you check the trunk before leaving the driveway. There’s no guarantee you’ll find a spare tire should catastrophe strike on the highway.

According to a report released by AAA this morning, more and more vehicles sold in the US are leaving the lot without a spare on board. The synecdoche for roadside assistance has been tracking the spare’s vanishing act for a while now, and reports that run-flat tires or inflator kits have replaced the "Oh no..." feature on 29 million vehicles over the last 10 years. The trend points north: 36 percent of model year 2015 cars were sold without the backup rubber, up from five percent in 2006.

Why is this eminently practical item getting tossed? It’s the fuel economy, stupid. As carmakers struggle to achieve maximum miles per gallon, they’re shaving off anything resembling dead weight to make vehicles lighter. A 50-pound backup system many customers never use is a tempting target, and comes with the added benefit of having more space to dedicated for passenger space or storage.

Automakers make up for the lack of a conventional spare with one of two options. They throw a tire inflator kit in the trunk, so customers can seal punctures and re-inflate their rubber. Or they equip their cars with run-flat tires, designed to stay inflated over limited distances after being punctured.

AAA argues neither's a great substitute. “We haven’t seen any decline in the calls for road service,” says John Nielsen, AAA’s managing director of automotive engineering and repair. “One way we can look at that data is to say maybe what’s failing on the tires is a catastrophic failure. In those cases, tire inflator kits aren’t going to help.”

In other words, a kit or run-flat may help if you drive over a nail, but if you suffer a blowout, you're hosed. After testing a variety of commonly found inflator kits, AAA found that they work great as long as you’ve just punctured the surface of the tread and the insidious bit of shrapnel remains in the rubber. Then you can use the kit to coat the inner wall of the tire with sealant before deploying the compressor to re-inflate it. Considering that’s a pretty specific use case, it’s no wonder AAA hasn’t seen a decrease in the requests for flat-related roadside assistance. And it turns out those devices, which have a shelf life of four to eight years, aren’t cheap to operate either.

“The inflator and the chemical all come as a kit and that’s expensive,” says Neilsen. “Then there’s the labor involved in cleaning out the tire, and the manufacturer requires that when you use their sealant you have to replace the entire pressure monitor. So all of that adds up to around $300.” Spare tires cost money too, but there are a whole host of temporary or compact options available for under $100, and unless you lose your jack and lug wrench along the way, those implements are likely to be one-time expenditures.

So are the downsides of losing the spare worth it? AAA says each four-pound inflator kit eliminates approximately 30 pounds of weight. Whether you think that’s a significant amount of heft depends on how you argue the point. AAA says the amount is negligible and that compared to the cost of replacing a spent inflator kit, the benefits of marginally improved fuel economy are offset.

Maybe you want to amortize what that weight contributes to fuel consumption over time, like the good people at Tire Rack did. They found that a 50-pound spare tire, wheel, jack and tools can reduce the vehicle's fuel economy by up to one percent. By their math, the driver of an SUV that gets 16 mpg can save 94 gallons of fuel over ten years. Even when gas is cheap, that's a nice chunk of change.

Automakers are endorsing that math as they continue to winnow down the number of cars leaving the line with spares, while AAA is beseeching those companies to side with “consumer interests” and re-implement the automotive stalwart that is the backup tire.

Regardless of whether you have run-flats, a full size spare, or an inflator kit in your arsenal, Nielsen says one of the best things you can do is just be prepared. If you’re all about the repair kit, make sure it’s in good working order. If you prefer keeping a spare, first of all make sure it’s actually in the car, then check periodically to see that it’s properly inflated. Most importantly: Know how the hell to use whatever tools you have on hand. The new AAA report also reveals that only 20 percent of drivers aged 18-34 even know how to change a tire. We’re willing to wager that 100 percent of those babes in the woods won’t be able to properly seal a tire either.

So go check your trunk, and keep your eyes on roadsides for millennials covered in rubberized tire sealant or standing aimlessly with a lug wrench in their hands. They probably need you to call AAA for them.