OPEC, IT DEVELOPS, threatens to accelerate the conversion of the United States to the metric system. That isn't OPEC's intention, of course, but American gasoline pumps can be adjusted to prices only up to 99.9 cents a gallon. The U.S. Metric Board calculates that it would be a lot cheaper and easier to shift them to liters than to add another decimal place in the mechanism that computes prices. The Metric Board is holding public hearings on the question next May 2 and 3.

For us who begin to have second thoughts about going metric-and to feel a certain sentimental reluctance to abandon gallons, pounds and inches-gasoline by the liter is not a reassuring prospect. The wine and liquor industry went metric a couple of years ago, and there were complaints that some of the shippers had taken the occasion to adjust prices sharply upward. The same thing happened on a large scale when Britain went to the metric system. Has Alfred Kahn considered the inflationary implications of unfamiliar weights and measures?

Four years ago Congress passed legislation establishing metric conversion as national policy. But it set no deadlines, and said that the process is to be voluntary. So far the changes have been most notable in big companies that do business across national boundaries-for example, the automobile industry, in which the transition to the metric system is far advanced. Soft drinks are now metric. Computers and chemicals are making rapid progress, and steel is coming along. Science and technology have always used metric units, and it is rational for manufacturing to join them.

But a country can afford to be rational only up to a point. Why not continue a dual system, with traditional units for people who have no particular reason to change them? There's no particular virtue in converting signs to say that it's 64 kilometers to Baltimore, instead of 40 miles. Only a zealot would insist on changing the national speed limit to 88.5 kilometers an hour. As for temperatures, the Fahrenheit scale is no more arbitrary than Celsius, and Fahrenheit has the considerable advantage of being far more familiar. In a world where much changes ineluctably and is unfamiliar, there's a sound case for taking a firmly reactionary stand on those happy occasions where the choice is harmless.