Up until now, it was only a guess, but researchers have now provided the factual basis: Plug-in hybrid cars drive less often electrically than they can, and as a result often use the internal combustion engine.

This is confirmed by a joint study by the Fraunhofer Institute for Systems and Innovation Research (ISI) and the International Council on Clean Transportation (ICCT), which is available to ZEIT ONLINE.

To put it in the words of critics of this type of drive: the charging cable of plug-in hybrid cars often stays in the trunk.

Hybrid cars combine a combustion engine with an electric motor and are therefore considered to be particularly economical.

The advantage is that when braking, they gain electrical energy, which is then stored in a small battery.

This current is used again when accelerating.

In addition, the internal combustion engine can work in the ideal speed range.

Fantastic consumption figures

Plug-in hybrid cars can be even more environmentally friendly because their battery is significantly larger and can be charged externally with electricity from the grid.

You can therefore cover commuting and everyday trips without using diesel or premium petrol.

The legislature - in this case the EU - recognizes this.

The EU procedure for determining consumption, however, leads to a lack of transparency and extremely low information from the manufacturer: A Volkswagen Golf GTE with a system output of 180 kW (245 hp) shows a consumption of 1.7 liters of premium gasoline per 100 kilometers on the paper.

For comparison: the classic Sport Golf without a charging plug has a consumption figure of 6.5 liters (149 g CO2 / km).

Real consumption two to four times as high as in the laboratory

In their study, Fraunhofer ISI and ICCT have compiled real consumption data from 104,709 plug-in hybrids from all over the world.

Most of them (84,068) come from the USA and Canada, but 1,385 privately used and 72 business-driven vehicles from Germany are also included in the balance sheet.

The figures from Germany are particularly interesting because only here are comparative values ​​for private and commercial use.

On average, the study says, fuel consumption is two to four times higher than the unofficial test cycle.

So it is not about the usual deviations between real and standard consumption, which are in the low double-digit percentage range, but many times over.

The analysis provides the explanation for this: "The real electric driving part is only half as large as the electric driving part according to the test cycle."

Actually, according to the test specifications, it should be 69 percent - but it's only 37.