I don't know about mainland Europe, but in the UK it really was exclusively about CO2 emissions per distance travelled, to the extent that Vehicle Excise Duty (the annual tax you pay on a car) was defined in terms of g/km of CO2 emitted. This happened in 2001 and wasn't changed until the wake of the emissions cheating scandals [1].
[1] https://www.gov.uk/vehicle-tax-rate-tables/rates-for-cars-re...