Does the amount of gas in the tank affect mileage?

Answer 1

For a simple answer: no. For a technical answer: not really.

There are other factors that affect mileage more than the weight of the gas itself. The biggest factors in mileage are the vehicle itself (size, tires, etc.) and external conditions (wind, rain, road gravel, pavement, etc.).

Only if you have a very large tank and it is full. This is only by way of the added weight.

It may seem that it takes longer to go from full to 1/2 than it does to go from 1/2 to empty. That is because the fuel monitor inside your gas tank does not go all the way to empty. It's range is Full to about 1/8th. So, full is full, and empty is really 1/8th. It is designed that way so you won't run out of fuel. But, dont run it that way. Fill back up when you get down to a 1/4 tank because the bottom of your tank has trash and other elements that can hurt your engine and clog your fuel filter.

For a simple answer Yes. The more weight in a car, the more petrol it takes to move the weight.

Even driving with lights on uses more petrol, nothing is free.