Exactly why gas mileage should be averaged over multiple tanks of gas. some pumps deliver fuel at a faster rate and cause the gas to foam and fool the shut-off into thinking it is full when it isn't. thereby looking like you got more miles on less gas, the next tankful takes more fuel and makes it look like you used more gas on less miles.
Even using the same pump, at the same time of day, same temp, and same route can be affected by humidity, barometric pressure, wind speed, temp of the tires, pressure on the gas pedal, weight in the car, etc. There are also variables inside the motor and transmission that can affect MPG as well. Averaging over at least 3 (or more)tank fulls will give you a better indication.
Even using the same pump, at the same time of day, same temp, and same route can be affected by humidity, barometric pressure, wind speed, temp of the tires, pressure on the gas pedal, weight in the car, etc. There are also variables inside the motor and transmission that can affect MPG as well. Averaging over at least 3 (or more)tank fulls will give you a better indication.
