CO2 calculations were part of my old job at FCA, so I may be able to shed some light.
The official US method, as bfeng mentioned, is to use the combined unadjusted (meaning, not label) fuel economy based on the EPA75 and EPA HWY test cycles. (assuming you aren't using the 5-cycle label or 3-cycle label calculations due to Litmus restrictions) (8887/(unadjusted combined mpg))=g/mi There's a formula that is applied that converts the dyno-tested fuel economy into the label fuel economy, hence why I mentioned 'unadjusted'. (for the US people, if you want to know more, the EPA shows all the raw data on their website)
That said, the EU used to use the WLTP and NEDC cycle, and those tests were not around in their current form when the coupes were made. There should be some sort of old car exemption or rule to get around this, as otherwise they would have to re-test the older cars with the new cycle to generate new consumption numbers. You generally can't just convert fuel consumption numbers between cycles because cars behave non-linearly on drive cycles.
Looking here:
https://www.acm.gov.pt/widget/-/com...-de-tributacao-do-imposto-sobre-veiculos-is-1
I see this line which seems important for your situation:
- Document the effective measurement of the level of carbon dioxide emissions by legally authorized technical center, where this element is not included in the respective certificate of conformity.
Note: CO2 emissions of used vehicles, resulting in an effective measurement for legally authorized service center, whose CO2 value below the constant oldest certificate of conformity of the vehicle of the same make, model and version, or in the case of this not on the available information, a similar vehicle, are not deductible for tax purposes, prevailing the value of the certificate.
This also seems relevant:
https://www.portugalresident.com/co2-emissions-tax/
If you really want to know what you will get, you can get it tested. Without knowing the cycles used to generate a consumption number, and making sure those same cycles are what the law is using, there's no way to tell, aside from testing. (or using a calibrated, forward iterating model with coastdown curves, engine maps, and efficiency maps for the trans, diff, and tires)
Alternatively, if you bring it in as a classic car, you may be able to avoid the whole thing:
https://www.angloinfo.com/how-to/portugal/transport/vehicle-ownership/classic-cars
A classic vehicle may be imported into Portugal tax-free provided:
- The vehicle is for private use only
- The registered owner of the vehicle is imported from another EU country where they have been resident for at least 185 days
- The vehicle has been used by its registered owner in their former country for at least six months
- DGV Certificate of Compliance Form Model 9 has been filled out and submitted to the DGV confirming the vehicle has undergone an inspection
- The owner has submitted a copy of passport, driver's licence, residency (or application), tax details and number and at least three years' original tax returns
- A certificate of cancellation of residence issued by the person's consulate
