And where did that definition say the word meant how fast something was consumed?
Here's the definition:-
2 a : the act or process of consuming <consumption of food> <consumption of resources
Was that food eaten in a week, or a month? Did we use those reasources in a day or a decade?
YOU said this:-
2 a :
Comsumption therefore implies a rate. Fuel Consumption will therefore always be "amount per something".
Which was an unfounded assertion not included in the definition you posted.