When can you omit using s after possessive nouns?

It seems that in some cases s is not used after possessive nouns, for example, you would not say Fuel's price went up instead you would say Fuel price went up. However, the sentence This car's price is too high sounds okay with using the ending s.

Can someone please explain the examples I provided from a grammatical perspective, and when using s can be omitted with possessive nouns?

Solution 1:

's or s' is never omitted with possessive (genitive case) nouns.

But some of your examples are not possessive, they are attributive.

Noun+noun compounds can work as attributive nouns where one noun modifies the other. See noun adjunct, attributive noun, or qualifying noun.

https://en.wikipedia.org/wiki/Noun_adjunct

A noun or noun phrase in English is only considered possessive when it is written with an actual apostrophe, as you would have with my child’s entertainer or a my children’s entertainer. Those are both possessives.

Car prices went up. - car modifies price (not all prices went up). *Car colours have been changed to reflect fashion."

This car's price went up. - this particular car 'possesses' a particular price. *This car's colour has been changed."

Fowler's Modern English Usage states in the section "Possessive Puzzles":

Five years' imprisonment, Three weeks' holiday, etc. Years and weeks may be treated as possessives and given an apostrophe or as adjectival nouns without one. The former is perhaps better, as to conform to what is inevitable in the singular – a year's imprisonment, a fortnight's holiday.