If you pay 18.4 cents Federal tax per gallon of gasoline then any cash transaction must be rounded up to the nearest nickel. A 12 gallon tank full would have 220.8 cents tax which would have to round up to 225 cents. The government would collect another 4 cents per tank in gasoline taxes without raising taxes.
If the 250 million registered US automobiles buy one tank a week that's over half a billion dollars of new fuel tax revenue. No wonder our president thinks it's time to do away with the penny. This same game can be played with many other taxes too.