The Sunday Times did a road test, driving a BMW 520d SE and a Toyota Prius from London to Geneva. The BMW used 49.3 litres of diesel, versus 51.6 litres of petrol (gasoline) used by the Prius.
While the BMW’s results are clearly respectable, the figures quoted in the Sunday Times article do not tell the whole story.
For a start, about 40% of the trip were on motorways, another 40% on B-roads and the rest in urban areas. A driving mix that includes only a token 20% of urban driving is hardly typical for usage patterns of most motorists in our largely urban / suburban societies (for example, 79% of the US population lives in urban areas, with most European countries having similar rates). This unusual mix seems almost purposely designed to ensure that the advantage of the hybrid drive train of the Prius would lie mostly idle: Driving at constant speed on a flat road, you are not going to see any real benefits from a hybrid system, which really thrives in stop-and-go rush hour traffic with lots of traffic lights, as most of us experience on the way to work or home.
Secondly, even with these skewed parameters, the BMW lost out on greenhouse gas emissions. It burnt 10.84 Imperial gallons (13 US gallons) of diesel, while the Prius used 11.34 Imperial gallons (13.6 US gallons) of gasoline. Because of diesel fuel’s 15% higher carbon content by volume, the BMW added 131 kg of CO2 to the atmosphere versus 120 kg by the Prius.
Personally, I see no reason why in the long-term efficient diesel engines can not be mated to a hybrid system and have the best of both worlds. Sure, it may not yet be cost-effective at current fuel prices, but things may look very different 10, 20 or 30 years down the road.