Cruise control and fuel efficiency I was driving up a hill with cruise control on and remembered reading an article stating that having cruise control on up and down a hill isn't the most fuel-efficient way of driving up and down a hill.
What is the most fuel-efficient way of driving up and down a hill?
I assume that fuel consumed can be directly argued in terms of power from friction, drag and gravitational forces.
 A: Driving slower improves mileage.
There are two main sources of energy loss when driving at constant speed on level ground. One is engine friction. The other is aerodynamic drag.
Drag is almost entirely caused by pushing air out of the way. Air has a surprising amount of mass, about $1$ kg/m$^3$. That air has to be accelerated from a stop to flow around the car and fill in the space behind. The mass of air pushed around in time $t$ is
$$m_{air} \propto Av_{car}t$$
where $A$ is the cross sectional area of the car. The average speed of that
moving air is
$$v_{air} \propto v_{car}$$
We can approximate the energy of that moving air as
$$E_{air} = 1/2 m_{air} v_{air}^2  \propto v_{car}^3t$$
So power lost to drag is $$P \propto v_{car}^3$$
At freeway speeds, this is bigger than engine friction. Slowing down is the best thing you can do for your mileage. Unless you want to consider buying another car will a smaller cross sectional area, or more efficient engine (E.G. electric).
Cruise control can help your mileage on the freeway. It can maintain a more constant speed than you might unless you constantly pay attention. And you can set it to a slower speed.

Driving up a hill requires energy to lift the car. $E = mgh$.
For a shallow hill, you get it back by coasting down the other side. You can maintain speed while backing off on the gas. But for a steeper hill, you get it back too quickly and must waste it in the brakes. You can get some of it back if you have a car with regenerative brakes.
Stop and go traffic is like driving up and down hills. You use gas to accelerate up to speed, and then waste it in the brakes.
You can help by anticipating when traffic is slowing down and backing off on the gas ahead of time. Cruise control isn't very good at this.
