Improved fuel economy standards for increasingly popular light trucks and passenger vans were proposed yesterday by the federal government for the 1980 and 1981 model years.

A separate report, meantime, showed that only 13.5 per cent of the nation's drivers use seat belts in their passenger vehicles. This finding "clearly support [s] the decision to mandate" air bags or passive seat belts starting with new cars in the early 1980s, said National Highway Traffic Safety Administration chief Joan Claybrook.

The fuel economy standards proposed by NHTSA would save an estimated 12 billion gallons of fuel over the expected life of the vehicles, according to the agency.

For the thing, the new regulations would extend fuel standards to three times the vehicles now covered by rules that deal only with vehicles of up to 6,000 pounds of gross weight by raising the limit to 8.500 pounds. Gross weight includes both the vehicles and its load.

NHTSA proposed an average standard of 16.2 miles a gallon for 1980 four-wheel drive vehicles and 19.2 miles a gallon for two-wheel drivers. The next year these requirements would increase to 177 miles a gallon and 20.5 miles a gallon, respectively.

Overall, Claybrook said yesterday's proposals would add from one to three miles a gallon better fuel efficiency than current models of vans, pickup trucks and four-wheel drive vehicles.

"Traditionally, vans and pickup truks were used for commercial purposes, but in recent years the public has purchased them primarily for passenger use." Claybrook told reporters.

However, she added, the Department of Transportation agency is not "inflexible" and will actively seek public comments on the proposals through Jan 26 and at a public hearing planned Jan. 16.

The proposed regulations, Claybrook estimated, would add $160 to the price of a typical van or light truck. But the estimated savings in fuel would be $712 over the expected life of each vehicle, she added.

One unusual aspect of the NHTSA proposals yesterday would include vehicles manufactured in U.S. plants overseas. If these so-called captive imports are included in the final regualtions. NHTSA said, the overall standard may be increased to 16.6 miles a gallon for four-wheel drive in 1980 (13 in 1981) and to 19.7 miles a gallon for two-wheel drive (21 in 1981).

Regular passenger car fuel economy standards mandate fleet averages of 27.5 miles a gallon by model year 1935.

The seat belt survey by NHTSA involved 84.682 drivers in 16 urban locations across the country. Conducted from August, 1976, through last March, the survey found that in cars with lap/shoulder combinations, belts were used by 15.7 per cent and in vehicles with lap belts only, they were used by 10.4 per cent of drivers.

"It is certainly discouraging to know that less than one in five American drivers are willing to take the simple life-saving step of buckling a safety belt," Claybrook said.

Among the study's findings were tnat:

More drivers of subcompact cars (29 per cent) and compacts (21 per cent) use seat belts tha n persons with intermediates (16 per cent) and standard sized cars (17 per cent).

Drivers with imported cars use seat belts more often than persons with American models.

More drivers in the West (27 per cent) use belts than in the East (12 per cent).

More women drivers (21 per cent) use belts than men (17 per cent).

More young drivers use belts (19 per cent) than persons over 50 (15 per cent).

More drivers use belts during evening rush hours (22 per cent) than any other normal daytime hours.

The lowest rate of belt use was among persons with such luxury cars as Chrysler's Cordoba, General Motors' Cadillac, and Ford ootor's Cougar and Lincoln (11.2 per cent to 14.9 per cent).