Knowing what fuel weighs is an essential bit of knowledge. While six pounds of weight per gallon doesn't sound like much, it's a weight that can certainly add up when you are hauling it. For vehicles that might be close to being overloaded, factoring in every last pound of weight counts and fuel shouldn't ever be left out of the equation.
In reality, it is water that is heavier than gasoline. A gallon of water weighs 8.3 pounds, a difference of 25%!
The vast majority of the time, this is through no fault of our own. It can naturally happen over time or is sometimes due to contaminated gasoline. There are signs that your fuel tank is partially full of water. You might suspect that there is water where it shouldn't be if you notice any of these symptoms:
One of the tell-tale signs of water in the fuel line is a rough acceleration. Any foreign liquids or solids throw off the air to fuel ratio in the combustion chamber. This will make the sensors not work correctly and cause them to under or overcompensate as they think there is too much or not enough fuel in the mixture.
When water enters the combustion chamber of your vehicle's engine, it can cause the spark plugs to misfire.
The water in the fuel line will impact how it idles, much in the same way it interferes with the acceleration. You'll notice that the engine doesn't hum as smoothly and sounds and feels like it can't keep a constant idling speed.
Steam coming out of your exhaust is a pretty good indicator of there being a good amount of water where it shouldn't be. While this could mean that the water is in the gas tank, it can also mean that your exhaust system has taken on water.
The sensors that detect whether your engine's fuel to air ratio is proper will not function properly if there is water in the fuel. This can cause the sensors to think the mixture is too lean. The result here can be a very slow acceleration.