Behind the glitzy song-and-dance sequences at the biennial Delhi Auto Expo, executives from automakers including Maruti Suzuki India Ltd., Honda Motor Co., and Ford Motor Co. are expressing concern about sudden changes in regulation.

In December, India's top court banned the sale of diesel vehicles with 2-liter engines or larger through March. A month later, the government brought forward stricter emission rules by four years. Together with other measures restricting vehicle use, they mark the most concerted efforts to address popular discontent triggered by the hazardous smog that blankets the country's capital. They were also imposed without prior consultation with automakers.

"Any surprises are never good for the industry," R. C. Bhargava, chairman of Maruti Suzuki India Ltd., said Wednesday. "It means investments become uncertain and everybody loses because of that."