[randolph]

randolph asked the Naked Scientists:

Dear naked scientists,

Does an automobile burn petrol at a slower rate when the tank is full? If so, is it due to the inertia created by the weight of the fuel?

Randolph Altergott
USA

What do you think?

[Bored chemist]

No. It uses more fuel if the tank is full, or if it carrying any other excess weight. It has to use energy to get the extra weight moving each time it accelerates and that gets wasted in the brakes when it slows down again.

[Kryptid]

Getting the car up to speed might require more fuel because of the excess weight, but wouldn't the inertia of that fuel also help it retain speed once it actually got there?

[niall]

A heavier car would be less susceptible to the effects of friction. But, a car with a full tank going at the same speed as a car with an empty tank has more kinetic energy (1/2mv²). This means that every time you accelerate to this speed the heavier you have to get all that energy from the petrol - the heavier car has more energy for the same speed so it uses more petrol to get there. The same thing applies from braking. To go from a certain speed to stationary you have to get rid off all the kinetic energy (to heat in the brakes). The heavier car has more kinetic energy lost going from a speed to stationary than a light car.

Your theory is like saying it takes less energy for you to push a super tanker really really hard to make it go 5 miles an hour than it does for you to push a shopping trolley to three miles an hour.

Definetly less fuel used when its empty - if you don't make much money like me you know that its more cost efficient to fill you car up £5 at a time!!

[Bored chemist]

A more heavilly loaded car also has more friction, The load at the wheel bearings is greater and the losses flexing the tyres as the car moves are bigger.
While the car is moving at a constant speed, the effect of the ecxtra weight is small, but since most journeys are nothing like that there's a big effect as nial explained.

[niall]

Yeh good point Bored chemist. I have no feel for how big this effect would be in comparison to the ones I talked about. We seem to be posting on all the same questions today.