General Motors Co. is facing calls to add air bags as standard equipment on its popular compact cars in Mexico, reflecting broader pressure on big auto makers to include basic safety equipment in their vehicles even when governments don't require it.

Four American consumer-advocacy groups, including Consumer Reports and Public Citizen, have sent a letter to GM Chief Executive Mary Barra calling on the company to make air bags a standard feature globally.

At least one of those groups said the company hasn't responded.

"Auto safety cannot only be for citizens living in wealthy countries," the U.S. consumer groups wrote in their letter to GM's Ms. Barra. "Yet GM's practice of providing some consumers with the best safety technologies, while not even providing air bags to others, strikes us as a morally indefensible decision."

Light-vehicle sales in Mexico have increased 19% through November, making it among the fastest-growing emerging markets for autos. GM, which is No. 2 in sales there after Nissan Motor Co., sells that country's most popular passenger car, the Chevrolet Aveo, but offers air bags only as an option, which many buyers choose to go without. Latin America's independent New Car Assessment Program recently assigned the Aveo zero stars in a crash test, calling the vehicle "unstable" with "a high risk of life threatening injuries."

Ms. Barra has been under pressure to clean up GM's safety image after a crisis over faulty ignition switches linked to dozens of deaths and injuries in the U.S. The crisis clouded the early days of her tenure and cost the auto maker billions of dollars in penalties and other damages. Praised by the Justice Department and others for taking swift action in the ignition-switch case, Ms. Barra has said the company will use its focus on safety technology to race forward in developing self-driving vehicles and gaining market share in certain emerging markets where it is underrepresented. Adding air bags as standard equipment in Mexico could be a relatively cheap step in that direction.

Independent health and safety organizations put the cost of adding a pair of air bags at between $100 and $150, and many auto makers in other markets have found that car buyers, who have an increasing choice of vehicles, generally welcome the devices. Like Mexico, many Latin American nations don't perform their own crash tests or have safety rules that mirror the standards in developed markets such as the U.S.