In America, we measure and weight everything with great accuracy. The gas pump is no different.
Every gasoline pump in America is checked on a periodic basis. That time scale is determined by local and state laws. It is usually based on a six month testing cycle.
An inspector, from the state tax division, will drive up to the pump, collect 10 gallons of fuel into a special container. The fuel is then weighted to determine the accuracy of the pump.
On every pump in America, you will find a small sticker from the state tax board. On that sticker will be the octane of the fuel, and a small box with a hand written number. All pumps in the US must maintain -10 to +10 teaspoons to be considered accurate enough for sales.
If the sticker on your pump reads -9 then you are getting 9 teaspoons less than a gallon. If it reads +5 than you getting 5 teaspoons more with every gallon you buy.
Most of us are creatures of habit, we buy our fuel at the same place and sometimes the same pump over and over. Minus 8 teaspoons over a year period can add up to a lot of fuel.
Check the sticker on your pump.
For the mathematically unchallenged. 1 US gal = 767.999 teaspoons
At -10 teaspoons, and $3.00 per gal, if you bought 20 gallons, you would loose about 60¢ every tank full
Now, having said all that, if you buy your gas just any old place, it probably averages out just fine.