Gains for investors in the nickel-a-gallon gas tax?

It's a nickel levy. And many automobile-owning Americans met the new federal tax on gasoline in April with a groan. But at least two Americans see those nickels, a few years down the road, as investment dollars in hand.

Fred Astman and Noble Trenham, co-chairmen of First Wilshire Securities in Los Angeles, look at the gas tax as an investment opportunity. They run a money management and investment advisory firm which manages some $200 million in discretionary accounts and has averaged an annual return of more than 25 percent. While the two are not proponents of higher taxes, they are bullish about a possible windfall for investors hidden in the new revenues slated to repair the country's roads and bridges.

So far, Mr. Astman and Mr. Trenham have singled out two public corporations which they believe will benefit from an estimated $4.3 billion a year for the next five years that the federal government, through allocation to the states, will pour into rebuilding the nation's run-down transportation network.

Their choices are relatively small operations - companies whose bottom lines are noticeably affected by any change in the economic environment. McDowell Enterprises Inc., is a Nashville-based outfit in real estate and various types of construction, including some 28 asphalt plants in the Southeast. The company's sales amounted to $91 million in 1982. But with construction margins low and interest costs high, it barely scraped by last year, producing a small loss - some $600,000 for the year. In the last three months the management picture has changed as a new chairman and president came on board.

The other company Astman and Trenham expect to benefit from the increased spending on roads and bridges is, by comparison, a relatively high-tech outfit. Until 1981, the Kratos Corporation, based in La Jolla, Calif., was a tiny ($50 million sales) firm that produced industrial controls and other technological equipment.

But in 1981, in a classic case of the minnow swallowing the whale, Kratos bought out Keuffel & Esser, the New Jersey-based company whose name is synonymous with slide rules. But K&E also sells surveying equipment and materials, drafting and media equipment, measuring tapes, and electro-optical instrumentation.

''Kratos is both directly and indirectly involved in the road-building area, '' asserts Mr. Astman, quoting a research report issued by Shearson/American Express which indicates that the increase in gasoline taxes could affect up to 75 percent of the company's sales.

''They are in a dozen areas involved with road building,'' Mr. Astman says. ''They have computer-aided drafting systems and reprographics and all sorts of things engineers need to repair and inspect bridges. . . . It is in the engineering know-how business.''

Kratos stock, which currently sells at around $14, is priced at only five times estimated 1984 earnings, which Mr. Astman calculates at $3 a share, up from a loss last year. Because of the consequences of sky-high interest rates on Keuffel & Esser, the company lost $1.72 a share in 1982.

''Last year (Kratos) spent $70 million buying K&E and the average prime rate was 16.5 percent,'' Mr. Astman notes. ''That meant very high interest payments.''

But he believes that ''when investors realize that the next five years look good for the company, it will command a higher multiple. This company should have a 15 multiple.''

The other company, McDowell, is a more direct road-building play. Astman points out that the firm's backlog is creeping up. Last year, he explains, when interest rates were high and little was spent on construction, the company's backlog was only $38 million. It has risen to $62 million this year, evidence that a pickup has already started.

While no stock analyst follows McDowell - including the regional brokerage houses in its own area - the firm's prospects have not gone entirely unnoticed. Last year two other firms took major positions in McDowell, and now own 41 percent of the firm's outstanding stock. Charter Company, an insurance firm based in Jacksonville, Fla., bought McDowell stock as an investment. The other firm buying the stock is the Southern States Group, which is in oil, coal, and computers. It installed McDowell's current chairman, Floyd Kephart. McDowell stock has been trading at about $10.

''These are about the two most boring companies we own,'' Mr. Astman says. But he adds that the environment they are operating in will be booming for the next five years. ''And that $4.3 billion figure for gas tax is misleading. The states are raising their gas tax, too, and doing the same thing. That money will go into this pot too,'' says Astman.