The idea certainly has its appeal. Deriving motor fuel from corn - or, someday, from otherwise useless agricultural by-products - could reduce our dangerous dependence on foreign oil and could provide us with a replacement for gasoline that burns cleaner and will never be exhausted.

Yet that dream has been no more than that for some three decades. It is a dream that has cost taxpayers billions of dollars with no discernible return on investment, unless your investment has been in Archer Daniels Midland stock.

Expensive new fuels are a better idea than conservation only if they pay for themselves, if they don't give billions of tax dollars to agribusiness giants such as ADM to reward them for making something we barely use - ethanol - out of something we have way too much of - corn.

We don't make ethanol from corn because it is efficient. Brazil does a lot better making it from sugar. And we don't use corn because it is environmentally friendly. Growing it sucks up huge amounts of energy and water and leaves tons of chemicals adrift in the ecosystem.

We make ethanol mostly out of corn because it is astoundingly plentiful, thanks to decades of heavy federal subsidies. Cheap corn, and the federal tax credit of 51 cents a gallon to ethanol brewers, have made it worth ADM's while to make ethanol. And The New York Times counts 39 new ethanol plants under construction.

None of this is market-based or ecologically driven. It has been calculated that growing corn for fuel is a net loss of energy, given all the fossil fuels burned in growing, treating, irrigating, fertilizing, shipping, drying and refining the crop.

It has also been figured that fueling large parts of the American economy with corn-based ethanol would suck up half of the nation's viable farmland as it raised the price of groceries.

The energy saved by serious conservation measures will far outpace any energy gained through subsidized, uneconomical technologies. Even those that seem as natural as pouring corn in the gas tank.