Editor’s Note: Our talk of cars tends to be related to a wide variety of topics like speed, looks, comfort and fuel mileage. Yet often overlooked — both by enthusiasts and everyday drivers — is one of the most important factors: safety. This week, we’re taking a deeper look at how modern vehicles are keeping you and your loved ones safe. We start here, with a review of how the National Highway Traffic Safety Administration tests cars’ performance in accidents — and how they’re getting it wrong.

Size matters in cars, and not just for carrying more passengers, extra fuel or having the space for a bigger engine. It’s both conventional wisdom and fact that heavier cars tend to be safer for their drivers and passengers during crashes. Yes, it’s a complex issue, and there are exceptions to the rule (a big, hulking SUV with a high center of gravity might be less safe than a well-designed sedan, for example) but generally, you want to be driving the Chevy Tahoe and not the Chevy Cruze if the two are destined to meet intimately on the road.

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And the stats prove the principle. Studies from the Insurance Institute for Highway Safety (IIHS) show that from 1994-1997, the death rate for occupants of sub-2,500 pound cars was 101 per million. The rate for 4,000 pound sedans was 85. 4,500 was 73. There’s clear strength in the numbers. This is true for a number of reasons, in both single car accidents and those involving multiple vehicles. Bigger cars give occupants more room to move — get into a wreck, and your body has the potential to get smacked around pretty hard by the interior of the car. The more interior space, the less likely you’ll be to smash your face against a hard A-pillar or a dash surface. Smaller, lighter cars also have smaller “crumple zones”, the areas of a car designed to absorb the force of an accident and diffuse it by “controlled deformation”.

But when it comes to crash test ratings that are widely recognized in the automotive industry and consumers, weight is not brought into the equation with the heft it deserves. The NHTSA uses weight classes in their annual safety report, but they’re broad ones: 3,500 pounds and up goes into one class, under 3,500 are split up into groups every 500 pounds, while SUVs, vans and light-duty pickup trucks get stuck in their own category. But two cars in completely separate weight classes can get the same rating, based on their laboratory conducted crash testing. For example, the 2014 Chevy Spark (automatic) weighs in at a Richard Simmons-like 2,368 pounds and gets an acceptable crash rating, while the Toyota Highlander gets the same rating but weighs in at 4,134 pounds. That’s a 1,766 pound difference, or very close to the weight of a Smart ForTwo. Which car would you rather drive at the other one?

Simple physics is proof in the pudding: Newton’s Second Law of Motion states that Mass multiplied by Acceleration equals Force. This applies to two cars of different weight smashing into each other.

Simple physics is proof in the pudding. Newton’s Second Law of Motion states that Mass multiplied by Acceleration equals Force. The Aveo’s force looks like this: 1074 kg (2,368 lbs) multiplied by 17.8816 m/s2 (40 mph) equals approximately about 19,204 kg of force. The Highlander generates 33,528 kg of force at the same speed. Though both of them get high marks in crash testing against a stationary object, it’s clear that the bigger, heavier Highlander will fare better in an accident.

Then there’s the difference between the conditions in the lab where crash testing is conducted, against static objects, and the actual paved real world, with real cars moving in all directions and a wide variety of deadly roadside obstacles. A smaller sedan like the Hyundai Elantra might do quite well in laboratory crash testing, but that only involves collisions with static objects. On the road, if the Elantra were in an accident with something much larger than it — say, a Hummer — there’s a potential for the smaller car to be pushed into other cars and stationary objects. Imagine Dwayne “The Rock” Johnson chest-butting Woody Allen in a crowded room.

And that’s exactly where crash testing and laboratory results don’t help very much. Though the data isn’t completely useless, it is easily less than sufficient in providing an accurate picture of how a car (and its passengers) will bear the brunt of an accident with another car of varying weight. There should be a reworking of classifications according to weight to make weight ranges in each class narrower. Furthermore, the awards given by the NHTSA and the IIHSA should ascribe scores according to how cars perform in specific weight classes and in collision with other moving vehicles of varying weight.

Until this problem is solved, all we can rely on is the data that’s out there — including a potential new car’s curb weight. It’s vital for new car buyers to do their own research beyond just current crash test result and into the realm of car weight. You’ll have to consider that a heavier car tends to get worse gas mileage; but putting safety first may well be worth the extra dollars at the pump.