Detroit's Big Three automakers gave a generally hostile reaction yesterday to legislation that would help two competitors meet federal gasoline-mileage standards.

A General Motors Corp. spokesman told congressmen that amendments backed by American Motors Corp. and Volkswagen of America would give "favorable treatment" to those companies at the expense of GM and other major U.S. automakers.

"GM has programs for future products that were formulated with the proper belief" that federal fuel efficiency standards wouldn't be changed suddenly to benefit a few manufacturers, said David S. Potter, company vice president for public affairs.

Standards set by the National Highway Transportation Safety Administration order stiff fines for companies that fail to meet fuel-efficiency levels that will rise gradually to 27.5 miles a gallon in 1985.

Potter and spokesmen for Ford Motor Co., Chrysler Corp., AMC, VW and Checker Motors Corp. testified before a House Commerce subcommittee considering various exemptions to current fuel standards.

The automakers reacted to the legislation as they were reporting that early-April slumped 24.4 percent below those for the same period last year.

The gloomy report meant the industry's year-long slump is continuing without letup.

Sales of domestic cars in the April 1-10 period reached 184,788 units, down from 217,290 in the same period last year. There were nine selling days in the period this year and eight last year.

All domestic automakers had sales declines except for Volkswagen of America, which managed to hold steady with last year's levels.

GM reported sales of 121,921 cars in the period, down 21.2 percent from last year on a daily selling basis. Ford sales of 38,755 cars were down 33.2 percent.

VWA said it sold 4,328 U.S.-built Rabbits, roughly equivalent based on daily rates to the 3,829 sold last year. American Motors sales were estimated at 4,400 cars, down 8.8 percent from last year. AMC reports sales figures only at the end of each month.

The early-April sales drop was anticipated following weeks of rebates and other special sales promotions.

So far this year, domestic car sales by all five makers are off 15.2 percent from last year.

GM is down 9.5 percent on sales of 1,264.973 units, Ford is off 27.6 percent on 467,141 and Chrysler is off 26.2 percent on 204,121.

Car companies now are required to meet overall fuel-efficiency levels for both their imported and U.S.-produced "corporate fleets," but VW and AMC want exemptions that would allow them to average many of their imports and domestic cars together.

Their spokesmen said at yesterday's hearing that the exemptions would enable the companies to produce more efficient cars and provide more jobs for Americans.

Potter and Ford spokeswoman Helen Petrauskas said the legislation would give an unfair competitive edge to VW and AMC after the Big Three committed substantial money and effort to meeting the federal standards.

The AMC-backed legislation would allow a U.S. automaker to include fuel ratings of up to 150,000 imported cars under certain conditions. AMC badly wants the amendment because of its recent merger with the French Renault Co. w

AMC spokesman Marvin W. Stucky said his company wants the exemption because current rules have had "the unintended effect of discouraging the establishment of new manufacturing operations which utilize foreign technology" -- AMC's situation with Renault.

The VW measure would allow averaging for foreign companies that began U.S. manufacturing after December 1975 -- a description that fits only Volkswagen of America

VW spokesman Philip Hutchinson Jr. said the Transportation Department and the United Auto Workers union favor the proposal, and he contended it would encourage foreign companies to start or expand U.S. production.

Subcommittee members gave no clear indication what they eventually will decide on the legislation, which is roughly matched in pending Senate bills. The panel's chairman, Rep. John Dingell (D-Mich.), said action should be forthcoming soon, however.

In two other developments yesterday affecting the troubled Chrysler Crop:

Moody's Investor Auto Service Inc. said it is lowering the ratings on Chrysler sinking fund debentures 8 7/8 percent due March 1, 1995, and the 8 percent debentures due Nov. 1, 1998, to CAA from B.

Moody's also said its rating of Chrysler's $2.75 preferred stock is being reduced to CA from CAA.

Moody's is also lowering its rating of Chrysler Financial Corp's senior debt to CAA from BA and of its subordinated debt to CA from B.

Moody's said these actions reflect the continued adverse operating results of the parent company which will make it extremely difficult for Chrysler to raise funds for its operating needs and capital expenditures.

Mitsubishi Motors Corp., which is affiliated with Chrysler, said it will supply Chrysler with engines for its new small cars beginning in May.

A spokesman for the Japanese automaker said the plan calls for shipments of 200,000 engines annually over five years.