New-car sales by the Big Four American auto makers rose almost 20 per cent in early March, led by a 33 per cent increase for Ford Motor Co. and record sales for General Motors Corp., the companies reported yesterday.

Ford vice president Bennett Bidwell said the results indicated the industry was recovering from adverse effects of the nation's bitter winter and entering the traditionally strong spring selling season.

GM vice president Robert Berger noted that company sales already had surpassed 1.1 million for 1977, only the second time in GM history that this mark has been broken this early in the year.

In the March 1-10 period, sales by the Big Four totalled 260,735 units, up 19.7 per cent from the same period of 1976.

GM said sales increased by 20.0 per cent to 156,335 units, a record for early March, and Ford reported it sold 68,249 units, a rise of 33.1 per cent from a year ago.

Chrysler Corp. sales rose 6.2 per cent to 34,203 units, while American Motors Corp. continued to slide, reporting sales off 18.8 per cent at 5,948 units.

There were these other auto industry developments reported:

GM is considering dropping its subcompact Chevrolet Vega and Pontiac Astre models at the end of this model year as part of a major realignment of its small car line, the Detroit News said yesterday.

GM officials refused to confirm the report, but Chevrolet general manager Robert D. Lund acknowledged the firm would "consolidate the bottom end of our product line, but I am not in a position to define right now just what we are going to do for 1978."

GM had cooling problems with Vega aluminum engines and had to replace thousands of the powerplants. GM now offers a five-year, 60,000-mile warranty on the Vega, well above the usual car warranty, in an attempt to boost confidence in the car.

A federal judge in Chicago dismissed a complaint that the Big Three auto makers conspired to fix prices to big fleet customers in 1970.

The lawsuit had been brought by a number of state and local governments and some businesses that regularly buy large fleets of new cars from General Motors, Ford and Chrysler. The plaintiffs had been asking for damages that could have totaled $1.5 billion to $1.7 billion.

Like two previous cases, the latest court decision found that, while the auto makers did make changes in their fleet pricing policies in 1970, there wasn't any evidence those moves were dictated by anything other than "lawful, independent competitive conduct."