and the drag coefficient is cd = 1/3 and the mass is mc = 1000 kg then the
special distance is: So “city-driving” is dominated by kinetic energy and braking if the distance
between stops is less than 750 m. Under these conditions, it’s a good
idea, if you want to save energy:

1. to reduce the mass of your car;
2. to get a car with regenerative brakes (which roughly halve the energy
lost in braking – see Chapter 20); and
3. to drive more slowly.

When the stops are significantly more than 750 m apart, energy dissipation
is drag-dominated. Under these conditions, it doesn’t much matter
what your car weighs. Energy dissipation will be much the same whether
the car contains one person or six. Energy dissipation can be reduced:

1. by reducing the car’s drag coefficient;
2. by reducing its cross-sectional area; or
3. by driving more slowly.

The actual energy consumption of the car will be the energy dissipation
in equation (A.2), cranked up by a factor related to the inefficiency of
the engine and the transmission. Typical petrol engines are about 25%
efficient, so of the chemical energy that a car guzzles, three quarters is
wasted in making the car’s engine and radiator hot, and just one quarter
goes into “useful” energy: Let’s check this theory of cars by plugging in plausible numbers for motorway
driving. Let v = 70 miles per hour = 110 km/h = 31 m/s and
A = cdAcar = 1 m2. The power consumed by the engine is estimated to be
roughly If you drive the car at this speed for one hour every day, then you travel
110 km and use 80 kWh of energy per day. If you drove at half this speed
for two hours per day instead, you would travel the same distance and
use up 20 kWh of energy. This simple theory seems consistent with the

ENERGY-PER-DISTANCE
Car
at 110 km/h
80 kWh/(100 km)
Bicycle
at 21 km/h
2.4 kWh/(100 km)
PLANES AT 900 KM/H
A380 27 kWh/100 seat-km
Table A.6. Facts worth remembering: car energy consumption. 