Advertised mpg doesn't match reality

By Julii Swindells 12 Oct 2011 | Comment
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Who would have thought? Car manufacturers finding ways to inflate advertised mpgs on new model cars? Could it be?

Actually, sarcasm aside, it's worth knowing why the manufacturer's stated/advertised figures are so unlikely to be achieved in reality, and what you can do to get your mileage figures closer to expectations.

Not quite lab conditions -->>

Explanation

  • Driving style and choices
  • Test conditions: quite unlike real ones
So much is said about driving style I don't want to add more here. But the test conditions are helpful to understand...

Test Conditions
Understandably have to be standardised; so they should be correct for comparing fuel economy between cars (in percentage difference terms). Unfortunately, they also tend to be standardised to conditions that are unlike how most cars are used in the UK. For instance:
  • No actual test on the road
  • No air conditioning, rough wind conditions, passengers or cargo, windows open, headlights or stereo on, and other common options that reduce fuel economy
  • For both urban and extra-urban tests, the environment (testing lab) is warmed to an ambient temp of 20-30 deg. C (makes fuel use more efficient, typical outdoor year round temp in UK = 10-12 C)
  • Cold engine start for urban test over 2.5 miles (25% of all UK car journeys are under 2 miles).
  • For extra-urban test, engine is run for a spell (warmed up) before the test commences
  • Non-urban test is over 4.3 miles, avg. speed = 39mph, Max speed = 75 mph

It is alleged that a car may use 40% more fuel in the first mile of travel than it does in subsequent miles (of the same journey). These short journeys also cause more engine wear (due to colder operating conditions, leading to repairs being needed sooner). Thus often using a car over short distances is especially expensive, and will drive mpg figures right down.

How to find out real figures and maximise your own MPG

Expect actual mpg to be 5-30% below the manufacturer's spec.
Consult one of several informal registers of actual mpg experienced (see links below); these data mostly from people who actively try to maximise their MPG, so give a good idea of what you should be able to achieve.
Follow good eco-driving habits to increase fuel economy. You may wish to investigate Hypermiling, too.

Links

SpiritMonitor (real numbers)
Fuelly (real numbers)
Honest John (real numbers, Daily Telegraph motoring expert, models from 2005+ only).
What MPG (manufacturer's specs)

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