What is the difference between "spare" and "reserve? Why do we use "spare tyre" and "reserve fuel tank" but not the opposite?
I think that an underlying pattern in the way many English-speakers use "spare" and "reserve" has to do whether the extra thing is an item to use or a quantity to consume.
The spare tire is an extra unused object to replace one that is currently in use, if the latter should for some reason be taken out of use. I can have a spare spoon, a spare copy of a book, and a spare person on my team.
I could have a spare tank, if it meant that I had an extra tank that could be used to replace a leaking or damaged one. When I think "reserve gas tank" or "reserve fuel tank", however, it is the gas or the fuel that I am considering to be in reserve. I could buy a spare tank to hold reserve fuel and then call it my reserve fuel tank. Fuel isn't a unitary thing or object that can break, or get lost, or get tired, and need to be replaced by an extra or spare, but is rather a substance that is consumed or used up and needs to be replenished.
I keep extra fuel in order to replenish my supply, and I keep my extra or reserve fuel in a tank- my reserve fuel tank. We can have a regular supply of sugar that we usually use, and a separate reserve supply (our reserve sugar) to replenish it, if it gets used up too soon. An army has reserve soldiers standing by to add to its supply of active soldiers, if too many regular soldiers are lost in fighting, or another need for more arises. I could keep a reserve bank account to keep extra or reserve money with which to replenish money in my regular account, if something happens to make me go beyond my usual budget and use the latter up too fast.