Question

Why does a car with 5 people in it use more gasoline than the same car with just the

Answer 1

because mass of 5 people in a car decrease the velocity of a car

thus, it takes more gasoline

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Answer 2

Any vehicle use more energy (fuel in this case) if its weight increase.
But a normal car weights around 1500kg, if you add the driver you have 1580 kg, if you add 4 people the overall weight would be 1900 kg, is not a big change but if you count all the energy you save avoiding the need to use 5 cars to move each one of those passenger, then you save energy instead spending more.

The consumption of any vehicle depends on the energy to accelerate that overall mass to certain speed, then you need to add friction from the wheels with the road (more mass = more friction), and then you need to add drag from the air which it depends on the aerodynamics and speed (it does not depend on the weight).
Each time you brake, you have to accelerate again, electric vehicles recover around 60%-70% of the energy waste in the braking.