Flat tires are still a common occurence.

Out of sight, out of mind?

Spare tires used to be such an important part of a vehicle that they were often incorporated in the basic shape of many models from the so-called golden era of design. For years, the Lincoln Continental’s trunk bore the distinctive, rounded shape of a spare – even when the fifth tire was moved to a less prominent storage location.

While a few classic SUVs, like the Jeep Wrangler, still give their spares prominent placement, those back-up tires are now more likely to be tucked under the trunk’s load floor – that is, if you have a spare at all. In many vehicles, from the sporty Chevrolet Camaro to the mainstream Ford Fusion, spares have been left out entirely.

By various estimates, somewhere between a third and a half of the new vehicles sold in the U.S. this year will come sans spare. Instead, manufacturers are opting for alternatives like tire inflator kits or run-flat tires, the latter designed to keep running for as much as 50 miles even if they lose pressure.

(Regulators in the US, Germany crack down on mileage, emissions tests. Click Here for more.)

Manufacturers content there are a variety of reasons behind this transition. For one thing, today’s tires tend to be more robust and better able to stand up to potholes, nails and other obstacles. Seldom needed, spare tires take up space and add weight – as much as 50 pounds or more on some off-roaders like the Wrangler.

Weight is the enemy of fuel economy and, in a push to meet new 2016 mileage mandates and the even tougher standards set for 2025, manufacturers are intent on cutting every ounce of fat. Getting rid of the spare is a big first step.

An increasingly uncommon sight: a spare tire.

“Automakers are facing increasingly-stringent fuel economy standards and the spare tire has become a casualty in an effort to reduce weight and boost miles-per-gallon,” noted John Nielsen, managing director of Automotive Engineering and Repair for the AAA.

Run-flat tires marked a big transition for the auto industry. The technology adds a super-stiff sidewall to the tire so that even if it loses pressure – even if it experiences a full blow-out – it can continue to run, albeit at a slower speed, far enough for a motorist to crawl home or find a service station. Run-flats are significantly more expensive and they deliver a much rougher ride than conventional tires, however.

So, many manufacturers have instead opted for tire inflator kits which weigh barely four pounds.

Basically, they use a special foam designed to seal off a puncture, then adding enough air for the tire to be driven on again. The AAA estimates tire inflator kits have replaced spares on 29 million new vehicles over the last decade, growing from 5% of the market during the 2006 model-year to 36% during the 2015 model-year.

But neither of these alternatives necessarily provide the perfect replacement for a spare tire, said AAA’s Nielson. The limited range of a run-flat can still leave a motorist stranded if they’re not relatively close to home or a shop that carries a replacement tire. And spare tire inflators can’t fix all punctures, especially blow-outs.

These alternatives are also expensive. A tire inflator kit can cost up to $300 per use. And the foam normally gunks up a wheel’s tire pressure monitoring system which must then be replaced with the old tire.

“Flat tires are not a disappearing problem, but spare tires are,” Nielsen warned, adding that without a traditional spare, the new systems can “leave motorists stranded at the roadside.”

The safety and travel group is calling on automakers to bring back the spare – even the mini-spare offered in millions more vehicles.

The AAA isn’t the only group concerned by this trend. The non-profit Consumers Union, publisher of Consumer Reports magazine, doesn’t necessarily warn shoppers away from spare-less vehicles but does note the alternatives won’t work in every situation.

It suggests new car buyers think about the issue – adding in a recent report that, “the best time to find out is in the showroom, not roadside, right after you’ve had a flat.”

Barring a sudden consumer revolt – which seems highly unlikely – the shift is almost certain to continue. In not that many years, spare tires could become as rare as running boards.

(Chevy Colorado Duramax gets nod as nation’s most fuel-efficient pickup. Click Here for the story.)