During Tesla’s most recent earnings call, CEO Elon Musk revealed that the Model S could have a 400 mile range before long, when the Model S and Model X switch to using the new battery cells which are currently in use with the Model 3.

“We’re rapidly approaching a 400-mile range for Model S. So, it won’t be long before Model S has a 400-mile range”

Elon Musk – Tesla CEO

Tesla hasn’t switched over to the newer battery cells yet and didn’t when the Model 3 was launched, instead the company worked on improving the efficiency of the existing batteries. At the moment, the Model 3 is rated for 373 miles of range in the US.

Source: Seeking Alpha