In Ireland road tax (paid annually) are based on engine capacity for (pre 2008 cars) and CO2 emissions for everything after that. (Engine size is a ok proxy for vehicle size)
https://www.gov.ie/en/publication/41c9cc-motor-tax-rates/#mo...
https://www.gov.ie/en/publication/41c9cc-motor-tax-rates/#mo...
The person that drive 1000km on a big SUV emits more than if he had bought a small an efficient car to drive the same amount. So that should be incentived.
> I believe a better system is to include the CO2 tax in the fuel/food price
I agree with that.