I didn’t find a topic about this, so…

I’m sorry but I can’t work out how the economy and emissions units work, couldn’t you use normal units (i.e. g/km of CO2 and mpg, l/100km or km/l)?

You can’t have a unit with a length (km, mile ect.) in it, to describe an engine that’s not moving. So until you fit the engine to a car you can’t have g/km or mpg.

Yup, without knowing how heavy the car is, what type of gearbox it uses, what gearing it has, how much aero drag it has etc. you can’t work l/100km etc out. The figures we give are so you can at least compare your engines to each other, so when building a car you know which one is more economical and by how much