Ever noticed that gas stations almost all advertise prices that end in 9/10? It’s a bizarre tradition no other industry shares. And it goes back 100 years to the first gas taxes.

The reason we don’t have to wonder about this bizarre pricing practice anymore is that WSYR news reached out to the Northwoods Petroleum Museum in Wisconsin. Museum founder Ed Jacobsen says the first time fractions of a penny appeared on gas prices was around 1919. That’s also when the federal government and some states passed a gasoline tax. And that’s no coincidence.

The first fractions on gas prices weren’t 9/10

Those were the good old days when a gallon of gasoline might cost you just 10 or 15 cents. So an entire penny would have been a steep tax. The taxes were often tenths of a cent and so gas stations began to write the extra fraction on the sign, tacking it right onto the end of the price.

In most industries, you couldn’t well charge a fraction of a penny. No one’s cutting a penny in half to buy a burger. But because you so rarely buy a single gallon of gas, all those tenths of a penny often added up to a whole penny. Gas stations round to the nearest penny for your final price.

As gasoline prices rose, gas taxes increased to whole pennies. But stations were still set up to display fractions of a penny on their signs. So by the 1950s, gas stations just increased that fraction up to 9/10. Today, it’s the gasoline equivalent of 99 cents, a psychological tactic to make prices look a bit more affordable while maximizing margins. You can see a great visualization of the history of gasoline prices in the video embedded below: