September 4, 2011

mpg vs. REAL mpg

Summer 2011. Gas prices at the pump have been around the $4/gallon mark, filling your tank can be a dizzying experience and "mpg" is the term of the day, featuring prominently on today's car ads. Why, even Hummers now come with a mileage specification.

But how is "The Fuel Economy" determined? And can you, as a car buyer, really trust those numbers from the car ads?

In the US, you can! Here, the fuel efficiency published by car manufacturers is based on tests that follow EPA guidelines that have been tightened in 2008. If you compare the EPA numbers to actual mileage reported by real-life drivers going about their daily business, you find that if anything the EPA number for "average" mileage is a very slight under-estimate of the actual mileage.

Elsewhere in the world, the official numbers for fuel efficiency are quite a bit higher than the actual mileage. Keep it in mind when roaming the internet for fuel efficient cars: The discrepancies can be stunning.

As an example, let us look at the 2011 Toyota Prius, because it comes with the same engine wherever it is sold. The table below shows both the "official" mileage and the real-life fuel economy for actual users. The latter averages to about 50mpg if you ignore the Dutch result (more about that below).

The EPA numbers are just about the same as the actual mileage in real life. Elsewhere on the planet, the official fuel economy varies from 60mpg, to an improbable efficiency of 83mpg according to the older Japanese 10.15 standard. After a promise like that, finding out the true mileage could be a bit of a letdown. So many dealers, at least those in the Netherlands, are very upfront about suggesting that you ignore the mileage on the nice display charts in the showroom, and proceed to tell you the street value.


Fuel Economy of Toyota Prius, selected locations

As Listed
(local units)
Translated into
Official Fuel Economy
US city / hwy 51 / 48 mpg 51 / 48
EU city / hwy 3.9 / 3.7 l/100km 60 / 64
EU avg 3.9 l/100km 60
Australia 3.9 l/100km 60
Korea 29.2 km/l 69
Japan 10.15 35 km/l 83
Japan JC08 30.4 km/l 71
Real-life Fuel Economy
US Dept. of Energy 50.3
US fuelly.com 49
US truedelta.com 52
UK Honest John 57 mpg(imp) 48
NL Dutch TNO study 5.8 l/100km 41


If you live in the US, you can contribute to the real-use efficiency data by volunteering to log your fuel use at one of several websites, including that of the US Department of Energy. The more participants, the better the accuracy of the numbers. Fuelly.com has a nice graphic interface that shows you not just the average but also the spread of mileage achieved. Truedelta sorts its data also by terrain (e.g. hilly vs. flat).

Honest John is in the UK, where they use imperial gallons, so you need to multiply all their numbers by 0.8327 to get mpg(US). For the Prius, the efficiency recorded in the UK, 48mpg, is a tiny bit lower than that in the US: apparently they still drive a little zippier there, even though the peak days of "road rage" are thankfully in the past.

The fuel economy from the Dutch study is way off. This is not because the team conducting the study made mistakes (TNO deservedly has a high reputation), but more because of the nature of the population they studied: drivers of cars leased by their employers. In Holland, this is a popular way to augment the employee's compensation package (initially, there were 240,000 cars in the study). So these drivers are (a) Dutch people (zero-patience, high-vigor drivers), (b) who work (higher stress), and (c) some of whom have their fuel expenses covered by their companies (no incentive to save). Pedal to the metal, guys.
If you must get your mileage number from this study, a guideline is that above a claimed C02 emission of 180g/km the measured mileage is within 10% of the "official" mileage, and above 230g/km it is within 5%. In any case CelloMom reckons that you can regard the measured mileage as the absolute minimum you will get in a particular car.
Self-reported mileage for Dutch drivers can be found at the Autoweek site: there the fuel economy for Prius is closer to 47mpg, but the variations are large, reflecting the range of driving style.


  1. Another nice website for user reported gas mileage is http://www.spritmonitor.de/en/ The website is originally German, but has English translation. I spot checked a few cars against fuelly and honestjohn, and it's pretty accurate.

    1. Thanks for the link! The more real-life data we can get, the better.


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