For example 8L/100km @ 80km/h
First thing I did was get the time I would take to complete 100km with my current speed
$$ \frac {100 \text{ km}}{80\text{ km}/\text{h}} = 1.25\text{ h}$$
then $$=\frac{8\text{ L}}{1.25 \text{ h}}$$
$$=\frac{800\text{ ml}}{75 \text{ m}} $$
$$= 106.6 \text{ ml}/\text{m}$$
Is this correct? Thank you
Yes, it's correct.
A one line method (with less thinking involved) would be $$\frac{8}{100}\text{L/km}\cdot 80\text{km/h}\cdot 1000\text{mL/L}\cdot\frac{1}{60}\text{h/m}=106.6\text{ml/m}$$