Tampering with pumps in an unauthorized nature does happen even though there are very regular checks that are done by the state's gas measuring technicians to ensure their accuracy. There is a very legitimate reason that explains this phenomenon though. You might wonder why you've never wondered about this or thought about this to yourself because it is very simple.

Most manufacturers put out estimates that the actual capacity of a fuel tank can vary as much as 3 percent from the rating of the tank capacity because of design characteristics and also because of the manufacturing process. Another thing is that the physics that are associated with the components that are built to monitor the emissions system and also the electronics on the indicators and systems of fuel.

Most manufacturers don't tell which parts of the fuel tank of the vehicle is that determine the capacity rating and this is very important to do. Also, what happens to these components when a vehicle is moving is a critical component in fuel capacity also.

There is a small area that is located at the bottom of the fuel tank that is known as the ?unusable volume? This is because it is below the fuel line and can't be used by the engine. This exists because it is physically impossible to build a gas tank in which it can be totally, 100% emptied out because if the fuel line was that low there would be air resistance in the line because it would be so close to the bottom of the tank.

Another part of the gas tank is the ?vapor head space.? This is the top most portion of the gas tank compartment. This is almost always left out when a gas tank is measured for fuel capacity because fuel can never go that high. It is the part above where the filler pipe leads into the gas tank.

Now, drivers sometimes fill their tank beyond the pumps automatic shut-off point. This results in fuel either raising up into the filler pipe and over flowing, or fuel being drawn into the vapor head space. Also, the surface at the gas station could affect this. If you are not on a level surface and the ground is sloped towards the side where the vapor head space is and not where the filler pipe is, you could be filling up the vapor head space before the automatic shut off point is reached. You should use extra caution when filling up at gas stations that don't have level surfaces because if this vapor head space is filled up it could cause a lot of damage to your car.

Your gas gauge sometimes reads over the Full reading and that is because tanks are supposed to be full much before the tanks actual capacity. Most pumps don't do this though and just fill it up to the top. Also, when your meter reads E, you should have at least one or two gallons of gas left. This is called your ?gas reserve.?