Smoggy air in the nation's cities may become significantly cleaner than official projections indicate, new discoveries about auto pollution suggest.

Planned new federal regulations to make gasoline less volatile--harder to vaporize--may cure a problem only now being recognized, some analysts say.This is the evaporation of gasoline from cars in use, called "running loss."

All cars have systems designed to control evaporation while parked. The Environmental Protection Agency has assumed the system also prevents significant vapors from escaping when the car is driven.

On a hot day, it now appears, the system can be overwhelmed and release five to 25 times more unburned gasoline than is permitted in exhaust.

The findings are so new that no one has had a chance to calculate their effect. but they suggest that plans for less volatile gasoline may significantly reduce this previously underestimated contributor to smog.

"Official predictions have been under-predicting" future improvement with the less volatile gasoline in use, said Tom Austin, a partner in Sierra Research of Sacramento, Calif., air pollution consultants.

Gasoline vapor is the major raw material for urban smog. Chemical reactions with other pollutants in sunlight produce ozone, which can make your chest feel tight.

In proposing a regulation to cut maximum summer gasoline volatility by 22 percent from the current voluntary standard, the EPA hinted at the new findings last August. But it said results were too preliminary to use and did not disclose numbers.

However, Hugh Shannon of Exxon Research Corp, disclosed numbers at an American Petroleum Institute briefing last week. He said EPA has results showing that "running loss" can reach 10 grams (about half a liquid ounce) per mile; his own tests show a maximum of 2 grams per mile.

The current limit in exhaust is 0.41 gram per mile.

While declining to reveal specific test results, Tad Wysor, an engineer in the Emissions Control Technology Division of EPA's Ann Arbor, Mich. lab, said of the 10-gram figure. "We haven't seen anything that it's wrong, but there's not a lot to back it up."

EPA wants to know whether this has been happening all along, or whether, as Shannon believes, only recently because of the rise of fuel-injected engines--more than 80 percent today compared with practically none in the mid-1970s.

Fuel injection systems continuously recycle uninjected fuel back to the tank, carrying heat from the engine compartment and making the fuel hotter.

This causes a pressure build-up which, with today's fuel, overwhelms the under-hood charcoal canister that captures and recycles the vapor.

Sam Leonard, director of automotive emissions control for General Motors Corp.'s environmental activities staff, said "You can't put a big enough canister under there" to capture all running losses, but EPA's volatility regulation "definitely" will eliminate them.

For several reasons, volatility has surged in the 1980's. That causes more evaporations and more ozone.

EPA has known that cars passing its stationary emissions test while using fuel of 9.0 pounds vapor pressure, typical of 1975, flunk badly on today's fuel, which may be 11.5 pounds or higher.

Leonard says that on one part of the test, GM researchers found 16 times as much emissions using 11.5-pound fuel compared with 9.0-pound fuel.

EPA last week published the annual list of areas failing to meet national ozone standards. The list rose from 62 to 68 names, and environmental groups said the data showed that smog is getting worse.

Most analysts have blamed the failure to reduce ozone levels more quickly on the failure of states and EPA to require stricter controls such as vapor recovery systems on gasoline stations.