Vast differences exist between the most fuel-efficient cars in the three major auto producing regions -- Europe, the US and Japan -- but the Toyota Prius is the only car to place among each area's top 10 least wasteful autos.

Relaxnews's analysis of data from three sources shows that diesel cars, despite the hybrid hullabaloo, provide consumers with optimal efficiency. Yet despite this fact, diesels are largely shunned by two of the world's three main areas that produce passenger cars.

As regards cars available in Europe, this year the diesel car rules, thanks to many technological advancements since its dirtier mechanical predecessors. With five cars rated more efficient than the Prius, according to data released by Auto News on October 2, Europe's diesel determination may be the superior choice.

In the US, hybrids far outpace standard gasoline models, according to the 2010 Fuel Economy Guide released October 15 by the US Environmental Protection Agency and the Department of Energy. All of the top cars in the US are hybrids, led by four entries from Ford.

Last April Japan's Ministry of Land, Infrastructure, Transport and Tourism released its top 10 list of the most fuel-efficient cars sold there in 2008, also topped by the Toyota Prius hybrid. Even more so than the US, the diesel market in Japan is already tiny and trending lower. However, even after standardizing their figures to comparable US numbers, Japan's standard petrol models achieve production equivalent to the hybrid vehicles in the US.

The discrepancy between fuel figures can likely be chalked up to the different testing procedures between the judging bodies.

Cars do undergo small alterations depending on their country of sale but usually to the standard options package and not to the engine. For example, in the Prius the diffrences between the US and Japanese models involve things like windshield wipers, seat size and other non-mechanical details.

Top 10 Most Fuel-Efficient Cars from the World's Biggest Producers

Rank Europe (mpg/kpl)
US (mpg/kpl)
 Japan (mpg/kpl)
1 Smart ForTwo diesel:  69/29
Toyota Prius hybrid:  49/21 Toyota Prius hybrid:  85/36
2 Ford Fiesta diesel: 64/27 Ford Fusion hybrid:  44/19 Honda Civic hybrid:  73/31
3 Seat Ibiza diesel:  64/27 Ford Mercury Milan hybrid: 44/19 Toyota Vitz:  56/24
4 VW Polo BlueMotion diesel:  64/27 Honda Civic hybrid:  43/18 Honda Fit:  56/24
5 Mini One diesel:  60/26 Honda Insight hybrid:  42/18 Toyota iQ:  54/23
6 Toyota Prius hybrid:  60/26 Lexus HS250 hybrid: 35/15 Mazda Demio:  54/23
7 Volvo C30 diesel:  60/26 Nissan Altima hybrid:  34/14 Smart ForTwo Coupe:  54/23
8 Volvo S40 diesel:  60/26 Ford Escape hybrid:  33/14 Toyota Belta: 52/22
9 Renault Twingo diesel:  59/25 Mazda Tribute hybrid:  33/14 Toyota Passo: 52/22
10 Toyota iQ diesel:  59/25 Ford Mercury Mariner hybrid: 33/14 Daihatsu Boon:  52/22

Note: mpg = miles per gallon; kpl = kilometers per liter

All efficiency figures are an average of the car's highway and city ratings.

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