Fuel consumption figures

What does the in-game fuel consumption figure represent? I’m guessing it’s some kind of combined cycle measurement?

I find I mostly get higher consumption figures than real-world cars would, even if I use something like 15:1 air-fuel ratio.
I am proficient in tuning and optimizing the engines, gearing and aerodynamics.

My point is everything on the engines feel extremely well modeled; I’m able to recreate the performance figures of almost
any real-world engine very accurately (if not for modern turbo boost curves…) but the fuel consumption figures seem a little high.

correct me if i’m wrong, but i do think it’s combined average fuel economy

If you go to detailed stats you can see the fuel consumption in one of the tabs.