DETROIT, SEPT. 5 -- General Motors Corp. is weighing a $40 million proposal by California and Texas natural gas interests to sharply expand the company's first-ever mass production of trucks that will run on natural gas, industry sources said today.

A consortium of natural gas producers and utilities is negotiating with GM to launch 1994 production of four types of truck engines and up to a dozen types of trucks, vans and buses designed and built to burn natural gas, sources said.

It was not clear how many vehicles would be involved. But if carried out, the plan would suggest an increase in the scope of GM's interest in compressed natural gas as an alternative fuel for cars and trucks.

GM would not comment today on any negotiations with natural gas interests, except to say that it does not expect any agreements in the immediate future. But sources in the natural gas industry described the general plan, which was confirmed by an aide to Garry Mauro, Texas land commissioner.

The plan calls for the automaker to pay $24 million and the gas industry $16 million to engineer the vehicles and engines to run on natural gas. It would involve all categories of GM vehicles weighing more than 7,000 pounds, sources said.

The 30,000 or so natural gas-powered vehicles now on U.S. roads were converted from gasoline-burning fuel systems and are considered inefficient.

Fueling cars and trucks with natural gas, which burns more cleanly than gasoline, is regarded as one way to meet the tough new clean-air standards being debated in Congress and implemented in states such as California.

The natural gas industry is hoping for major commitments by GM and other auto companies to build natural gas-ready cars in the factory. But the automakers are reluctant to spend much money on such projects until they know the final shape of a clean-air bill nearing completion in Congress.

However, GM, the Texas General Land Office and 10 natural gas utilities in California, Texas and Colorado announced in July that the automaker would begin producing at least 1,000 Sierra pickup trucks in early 1991 that would be designed and built to run on natural gas. They are intended for sale to fleet customers, including the utilities themselves, that have refueling facilities.

At the time, GM -- which has favored methanol and so-called "reformulated" gasoline as solutions to the auto-emissions problem -- said the Sierra project was a first step in a natural gas development program. The $40 million proposal is the first indication of what the next steps might be.

The surge in gasoline prices that followed Iraq's invasion of Kuwait on Aug. 2 has further provoked interest in natural gas as an auto fuel. GM said that it has since been deluged with proposals from other natural gas interests.

"We plan to have a contract in place by the first of October," a natural gas industry executive said today. "But if it shows up in the {news}paper, it's very likely that GM would cancel the program. They're that sensitive about it."

The executive said that GM does not want to lock itself into the program until it sees the final shape of the clean-air legislation.

However, a GM spokesman said that the clean-air bill has nothing to do with GM's plans and that none of its negotiations on natural gas projects will be completed as early as Oct. 1.

"There's nothing that imminent, nor would we postpone anything because of the clean-air bill," said Thomas Klipstine, a spokesman for GM's truck and bus group.