Three dollars and ninety-nine point nine. Always that stubborn fraction. It is not a tax. It started in the Great Depression as a way to soften new gas taxes and somehow never left. The quietest pricing trick in American retail is still on every major sign.

Why do gas prices always end in nine-tenths of a cent? That stubborn .9 on every pump from Maine to California is not a tax and not required by law. It is a Depression-era pricing habit that became permanent psychological furniture.

In the 1930s, stations began pricing in tenths of a cent to soften the blow of new federal and state gas taxes when a gallon cost ten to twenty cents. Drivers flying past big roadside signs only read the leftmost digits, so ending in .9 made the price feel lower. The practice locked in by the 1950s and never left.

Today the federal gas tax  ...