Some friends are looking to change car soon. They are obsessed with emissions issues and look for low MPG cars with low carbon. Their current car (Smart) does not come remotely close to its real MPG. Its predecessor (Skoda Fabia 1.9 TDI) always did as claimed.

To obtain real emissions, would a reasonable proxy be to uplift the carbon value by claimed MPG over Real MPG? If so their Smart is only marginally better on emissions than their old Fabia which achieves 101% of its claimed MPG figure.