Caveat emptor! Things are not always what they seem. Or, WHOA!
Right. CelloMom is in shock. Bowled over. Blown away. Consumers have long questioned the fuel efficiency as quoted by manufacturers, who are always careful to state that actual mileage may vary and depends on driving style, or something to that effect. Today, CelloMom has found a study that uncovered the scandalous extent to which quoted efficiency can differ from actual efficiency.
The Dutch, who are acutely cost-conscious, undertook a project to measure actual mileage achieved by real drivers. The research was done by TNO, the Dutch Organisation for Applied Scientific Research, whose research team analysed mileage accrued by 130,000 drivers of leased company cars, and the fuel they bought to drive those miles. They found that the test cycle used under EU guidelines consistently overstated the fuel efficiency (by 16% on average), and that the discrepancy was largest for the most fuel-efficient cars, up to an astounding 45% difference.
To qualify that (nothing, but nothing, is ever truly straightforward): The extremely aggressive driving style generally employed by the Dutch does not favour fuel efficiency. Even New York and New Jersey drivers are relatively mellow compared to Dutch ones. The research finds that losing the aggression improves your fuel efficiency by up to 10%; so this explains only part of the discrepancy between actual and quoted efficiency.
Consider the case of the VW Golf, 2.0L TDI. In various parts of the world, its city/highway efficiency (mpg) is quoted as follows:
30/41 US EPA
The average mileage quoted in Europe is 55 mpg, whereas the actual mileage found by the TNO team is 39 mpg, a 41% difference. Keep this in mind when pondering fuel efficiencies quoted on European websites! --On the other hand, the good news is that the EPA guidelines for determining fuel efficiency reflects real-use conditions more closely. Indeed, the US EPA numbers for fuel efficiency tend to be lower than the actual efficiency.
Not all is lost: Travelcard, the fuel credit card company that commissioned the research, has shared the results on http://www.werkelijkverbruik.nl/ ("actual mileage"). The site allows you to specify year, make, model, and fuel type of the car you're interested in and shows the mileage achieved by real (Dutch) drivers as well as CO2 emissions. If, like CelloMom, your driving style has mellowed since you've acquired precious cargo to transport (children, cellos, whatnot), you can consider these numbers as your worst-case fuel consumption.