173 Starting and driving
Economical drivingFactors affecting fuel consumptionThe fuel consumption of a car is greatly
affected by driving conditions, climate, road
conditions, speed, driving technique, and
so on.Breaking-inFuel consumption may be somewhat
higher during the break-in period (the
first 3,000–4,000 miles (5,000–7,000 km)).Weather conditionsSummer temperatures give better fuel
economy than winter ones. The difference
can be as much as about 10%. Consump-
tion increases in cold weather because the
engine takes longer to reach its normal
operating temperature. The transmission
and wheel bearings also take longer to
warm up. Even distance affects consump-
tion: short journeys of 3–5 miles (5–8 km)
do not give the engine enough time to
reach its optimum temperature. Wind con-
ditions also have a marked effect on fuel
consumption.How to read the diagram:
If fuel consumption with the engine at
normal temperature is 28 mpg (10 l/
100 km), the actual fuel consumption
3 miles (5 km) after the engine has started
from cold will be:
– 23.5 mpg (12 l/100 km) at outside temp.
of 68°F (+20°C) (increase of 20%).
– 17.7 mpg (16 l/100 km) at outside temp.
of 32°F (0°C) (increase of 60%).
– 14 mpg (20 l/100 km) at outside temp.
of –4°F (–20°C) (increase of 100%).
It is clear from the diagram that both dis-
tance driven and outside temperature have
a significant impact on fuel consumption,
after a cold start.
If the car is normally driven on short jour-
neys, 3–5 miles (5–8 km), for example to
and from work, the average consumption
will be 60–80% above the normal level.
Driving techniques To obtain the best running economy, with
regard to fuel consumption and general
wear, the car must be serviced regularly.
High speed, unnecessary acceleration,
frequent braking and much changing
down all give higher fuel consumption.
Frequent gear changing in traffic, starting
in cold weather, the use of studded tires
and roof racks, and towing a caravan all
increase fuel consumption.
Do not warm up the engine while the car
is stationary.
If idling, the engine will take much longer
to reach its optimum temperature, and Percentage increase in fuel consumption
of engine starting from cold compared to
engine at normal temperature.
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