Question

why should we conserve fuel? how

Answer 1

We should conserve fuel because it is made out of natural resources, and the naturals resources are limited. Thus, the natural resources will famish and we will not have enough natural resources. We can save fuel by carpooling, using public transport etc. 

Answer 2

Because the Fuel is a non-exhaustible or non-renewable natural resource and if we will use them continuously then there will be lack of fuel or no fuel will be on earth since these are limited and our future generation will be unable to use them, since it is now a essential part of life.
We can conserve them by  judicious use of them that we can get benefits from these resources generation by generation. Use it by community in a judicious manner like public transport this is the best example to conserve the fuel(resource).