Tesla Motors this week trumpeted the results of a federal auto test of its electric Model S sport sedan, calling it the safest car in America.

But what went unnoticed was that the Palo Alto automaker had little relevant competition – federal safety regulators don’t test most luxury cars. So the Model S may well be safer than all cars costing $50,000 less. But whether it's safer than direct competitors from Audi, Mercedes or BMW remains a mystery.

Such cars aren’t often rated because crash testing is hugely expensive, and even more so when luxury cars are involved. The government destroys lots of vehicles and high-tech test dummies in controlled scientific conditions.

That’s why NHTSA tests only about 85% of new models. And that's up from about 65% just a few years ago.

It’s a logical approach. The agency prioritizes vehicles that sell in great numbers – the Toyota Camry, for instance, or the Ford F-150 truck – along with new or overhauled models and those with new safety equipment.

“This allows us to provide star rating results that best represent what is actually being purchased in the marketplace,” NHTSA said in a statement.

Tesla will sell just 20,000 of the 15.5 million vehicles Americans will purchase this year, for prices that can run upwards of $100,000. But the Model S got crash-tested because of the new technology factor: all-electric drivetrain, the agency said.

Tesla’s direct rivals – such as the Audi A8, the BMW 7 series, and the Mercedes-Benz S-class – did not meet the agency’s selection criteria for crash-testing.

A manufacturer can pay to have a vehicle tested. But why spend that money if you are only going to sell 10,000 cars and the model is already loaded with safety technology far exceeding government standards?

Mercedes, for example, loads up its S-class with features such as a forward collision avoidance system, which alerts the driver and automatically triggers the brakes when the car is about to strike another car, person, or telephone pole.

Tesla also boasts that the car scores better than any other vehicle in the rollover test, because of its heavy battery pack under the floorboard.

But that may not much matter, relative to Tesla’s competition. As the chief executive at one automaker told me, it’s not exactly easy to flip any of the big German sedans, either. The likelihood of a rollover in any of the big luxury models, the executive said, is so small as to make rollover crash-test comparisons irrelevant.

Maybe some other crash-testing organization -- say, the Insurance Institute for Highway Safety -- could step up and wreck the big German sedans. Maybe throw in a Lexus or a Jaguar, too. That way we'd all know how much safety we're getting the next time we drop $100,000 or so on a car.

ALSO:

  tops small-car crash test, fails

to pay owners for faulty MPG rating

Review: 2014 Chevrolet Corvette Stingray Coupe shifts from tradition

Follow me on Twitter (@LATimesJerry), Facebook and Google+.