What would happen to the economy if we all came together (a large majority) and decided to boycott using fuel for atleast a week?

That's the thing. They know we can't.

If many people are not prepared to modify their social behaviour to save others' lives during a pandemic (unless forced to my government regulations), you'd never get this one out of the starting gate (not much chance itd be enforced by government regulations!).

Fuel prices are determined by supply and demand.

Lower the demand, supply rises, prices will go down. That's why it dropped during the pandemic.

Of course, there will be a price spike when we all fill up afterwards.

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Consumerism and capitalism baby. All the models know and have geared us towards consumption and hence we are in this state. If lets say humanity came together and said hey lets consume less and use technology to get sustainable lifestyles the world would be a better place. But no one will.