Possession Is Nine-Tenths Of The Law... ...unless that changes

I recently downloaded a gas pricing widget for my Mac that tells me where the cheapest gas in my area can be found. The information is provided by GasPriceWatch.com, and it’s a simple way to shave a few pennies off every tank of gas. What caught me off guard was the way gas is priced: $x.xx.9/10ths. 9/10ths? We’ve all seen it on the signs and on the pumps, but you never really process it, you’re just aware of the overall price. So what’s up with 9/10ths? What other industry is allowed to price their product to the 9/10ths of a cent?

The bigger question is if you were to buy only one gallon of gas at say, $3.42 and 9/10ths, exactly how would they charge you for one gallon? Wouldn’t they have to round down? If they round up, then isn’t the price per gallon actually $3.43?