A newer version of the same model car has higher CO2, why is this?

I have just purchased a 2021 (21 plate) Ford Focus ST-Line 1.5 diesel 120hp and part exchanged a 2019 (19 plate) Ford Focus ST-Line 1.5 diesel 120hp because it was approaching a 5 year old cut off that my company requires.

Getting the tax and insurance for the newer vehicle I noticed that the g/km of CO2 has gone up from 103g/km to 118g/km despite it essentially being exactly the same engine and in fact the older car didn't require 'ad-blue' but the newer one does. The car tax I am paying as a result is higher despite thinking I was buying technically and performance wise, exactly the same car. Can you explain why the CO2 has risen so much on the newer one?

Asked on 21 November 2023 by Simon Reavell

Answered by David Ross
There are a couple of reasons this may be the case. One is that the CO2 performance of your vehicle has been revised, something which manufacturers are frequently required to do throughout the lifecycle of a model. The most likely issue however is that your 2019 car's CO2 performance was measured under the old NEDC official testing regime, which produced better results. Since then the WLTP testing regime has been introduced, which is designed to better represent real-world driving conditions and produces results which are more likely to be replicable. Almost every car produced higher fuel consumption and CO2 emissions as a result.
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