The two models did not fail any of the crash tests, but did fall short of meeting all the criteria needed to be rated a "Top Safety Pick" by the IIHS.
For the Model S, one crash test where the electric car did not get the highest rating of "Good" was the small overlap front crash where it was given an "Acceptable" rating by the IIHS.
In that test, the IIHS says the Model S safety belt allowed the dummy's torso to move too far forward so the dummy's head hit the steering wheel through the airbag. The IIHS also raised concerns about potential leg injuries for drivers involved in small overlap front crashes.
"Neither of these (potential injuries) were so high that we would expect life threatening injuries, but they are too high in our opinion to get "Good" ratings for those body regions," said Dave Zuby with IIHS.
Tesla believes the tests do not reflect the safety record of the Model S.
"We are committed to making the world's safest cars, and Model S has previously received a 5-star safety rating from the National Highway Traffic Safety Administration and a 5-star rating from Euro NCAP. Model S still has the lowest ever probability of injury of any car ever tested by NHTSA," said a Tesla spokesperson.
The IIHS notes Tesla has made production changes to the Model S to improve its ability to protect passengers and that the updated model will be tested at some point by IIHS.
Tesla believes those improvements will change the conclusions of the IIHS.
"We expect to receive the highest possible rating in every category, making Model S eligible for the IIHS Top Safety Pick award," said a spokesperson.
For BMW, the i3 received its "acceptable" rating in head restraint and seat evaluation, which measure how the vehicle will do protecting against neck injuries in a rear crash.
"We expect that the BMW engineers are working on improving the rear crash protection of the seats and look forward to testing those when they're available," said Zuby.
While the Model S and i3 fell just shy of getting the best rating possible, the IIHS awarded the Chevy Volt and Toyota Prius Prime as Top Safety Picks.
Zuby says the crash tests proved electric vehicles could provide safety while also offering the benefits of lower emissions.
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