Measuring as close to the source as possible is an iron principle of the conscientious engineer. In this respect, it seems completely plausible at first glance to determine the CO₂ emissions from the exhaust caused by a car. And indeed that was a good idea in a completely fossil-fuelled past, when an engine was always an internal combustion engine and every liter of fuel was petroleum-based. The fleet limits for new vehicles, which have been in effect since 2012 and then gradually tightened, promoted new technologies and actually led to lower greenhouse gas emissions, at least per kilometer driven.
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