The second round of "downsized" cars from General Motors Corp. will hit dealer showrooms in October - and the impact on the average American car buyer is likely to be broader and more dramatic than the first round a year earlier.

Arriving this fall are the totally new and strikingly smaller "intermedites" from GM. They are the Chevrolet Chevelle Malibu and Monte Carlo; the Oldsmobile Cutlass; the Pontiac LeMans and Grand Prix; and the Buick Century.

They will be lighter by 550 to 750 pounds and shorter by 12 to 18 inches. They will be six or seven inches narrower, a result of the doors being less rounded, but almost exactly the same height.

The wheelbases will be four and eight inches shorter, and the standard and optional engines will be substantially reduced in size.

The standard engine in the 1978 model Buick Century will be a new 196-cubic-inch V6, for example.

Perhaps the most important new numbers for car buyers will be the fuel economy figures for the '78 intermediates. They will boast in an average composite mileage, measured by the EPA, of 22 to 24 miles per gallon.

Coming in the mid-size car range, the new ones will probably impact on the public harder because, even "downsized," the big cars buyers think they need. In may respects the dimensions and performance of the new GM intermediates compare more closely to European cars like the Volvo or Volkswagen Dasher then they do to previous American-made cars.

One major problem looms for GM: the new intermediates will in most respects be somewhat lighter, shorter and narrower than the GM compacts. The Chevrolet Nova, Pontiac Ventura, Oldsmobile Omega and Buick Skylark will be essentially the same cars for 1978 that they are this year.

The 1978 Chevrolet Malibu - the name Chevelle will be dropped entirely this fall - will weigh 3,100 pounds, have a 108-inch wheelbase, and measure 192 inches overall. It will be 70 inches wide.

This year the Chevelle Malibu has a 112-inch wheelbase in two-door versions, and 116 inches in four-door versions. It weights 3,600 to 3,800 pounds, measures 205 to 210 inches to overall and is 77 inches wide.

The "compact" Chevrolet Nova has a 111-inch wheelbase, measures 197 inches overall, is 72 inches wide and weights about 3,350 pounds. Those dimensions remain the same for the 1978 Nova.

Similary, they hold the same for the 1978 model sister cars to the Nova: the Omega, Skylark and Ventura. But the 1978 LeMans, Cutlass and Century Shrink as greatly as the Malibu does.

That means all the 1978 GM compacts will in most instances be bigger than the intermediates.

The giant automaker thus is in roughly the same situation it has been in for the 1977 model year, where the full-sized cars frequently boast smaller dimensions than the present intermediates in everything but passenger room.

It was for that reason the auto industry here watched the pricing and launching of the 1977 GM big cars so carefully nine months ago.

GM held off pricing those cars for several weeks last fall, not including them in the initial lists of new car prices.

Then when GM priced the big cars, they got the same price increase the other GM products received.

The company counted on the price differential between the lines to make up the differences lacking in inches and pounds of steel. The average 1977 Chevrolet Impala sells for about $6,300, the Chevelle Malibu for about $5,300.

That strategy apparently worked. According to Ward's Automotive Reports, for the 1977 model year GM's downsized big car sales are up about 24 per cent while the total for all GM cars is up roughly half that.