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I'm talking about regular combustion engines that are in small 4-door sedans (like Saabs, Fords, and the like). Part of me thinks that the half tank would provide better mileage because there is less pressure in the gas tank to push the gasoline through the fuel line. On the other hand, the fuel could be easier to "suck out" since there might be equalized pressures from the gas tank to the fuel line. I don't know anything about pressure levels inside a gas tank or even how the gas gets fed to the engine. Pure speculation.

It makes no difference.

YOU MEAN THE HIGH PRESSURE PUMP DOES NOTHING? I can tell you understand the principal but there is a difference in many liquids because for one thing gas is compressible while other things like water is not.

The gas tank is not pressurized. Most modern cars use an electric fuel pump located inside the gas tank to pressurize the fuel lines and move gasoline from the tank to the engine. The only possible effect of having less fuel in the tank is that the car would be a few pounds lighter, thus making it slightly easier for the engine to pull the weight of the car and driver. The difference would barely be noticeable.

A half tank - the car is lighter by 20kg or so and will take less energy to accelerate or drive uphill.

The difference is likely miniscule - you'd probably get better improvement by washing all the dirt off or emptying all the junk out of the trunk.

Gas isn't fed by gravity anymore. It's picked up in the tank by the fuel pump and pushed to the intake on the engine. It's delivers a constant flow at a constant pressure. Whether the tank is full or not makes no difference.

You do get better mileage with a half tank than a full tank. A half tank weighs less than a full tank. But in a car, the increase in mileage is so minuscule you probably won't notice it. You'd notice it more on a long haul flight. A Boeing 777 going from New York to Paris, on take-off weighs significantly more than on landing after it burned all that fuel going across the ocean, so ignoring any winds, to go the same speed the plane would need more thrust an hour after take-off than it would an hour before landing. On a much, much smaller scale, that's what happens with your car as you burn gas.

Gas isn't fed by gravity anymore. It's picked up in the tank by the fuel pump and pushed to the intake on the engine. It's delivers a constant flow at a constant pressure. Whether the tank is full or not makes no difference.

You do get better mileage with a half tank than a full tank. A half tank weighs less than a full tank. But in a car, the increase in mileage is so minuscule you probably won't notice it. You'd notice it more on a long haul flight. A Boeing 777 going from New York to Paris, on take-off weighs significantly more than on landing after it burned all that fuel going across the ocean, so ignoring any winds, to go the same speed the plane would need more thrust an hour after take-off than it would an hour before landing. On a much, much smaller scale, that's what happens with your car as you burn gas.