This is the 17483rd most frequent Russian word.
топливной
"Топливной" means "fuel-related" or "fuel."
Here, 'топливной' is used as an adjective modifying 'бак', indicating that the tank is related to fuel.
Топливной бак автомобиля был полностью заполнен.
The car's fuel tank was completely filled.
The word 'топливной' serves as an adjective qualifying the noun 'системе', describing the system as related to fuel.
В топливной системе произошла утечка.
There was a leak in the fuel system.
In this instance, 'топливной' modifies 'эффективности', emphasizing the efficient usage of fuel as a concept.