How is road tax calculated based on CO2 emissions?

Question

Accepted Answer

For most cars registered from April 2017 onwards, CO2 emissions only affect the first year of road tax — after that, most vehicles move to a flat standard rate, regardless of their emissions figure.

How it works:

First-year rate — based on the car's official CO2 emissions at the time of registration, banded from £0 for zero-emission vehicles up to several thousand pounds for the highest-emitting cars
Standard rate (year two onwards) — a flat annual fee that applies to most petrol and diesel cars regardless of emissions, with a separate (often lower) rate for hybrids and alternative-fuel vehicles
Electric vehicles — historically exempt, though rules around EV taxation have been changing, so it's worth checking the current rate for the registration year in question

Cars registered before April 2017 follow an older system, where CO2 emissions determine the ongoing annual tax band for the life of the vehicle, not just the first year — so two similar-looking cars can be taxed very differently depending on which side of that April 2017 cut-off they were registered.