"Full" coverage is somewhat of a catchall which also includes third party coverage (liability to others.) Presumably you're talking about first party insurance for collision or comprehensive type damage.
The measure of coverage is determined by what the owner can afford to lose, or to pay as a deductible in case of a loss. This has to be weighed by the amount of deductible the owner can afford to pay as well as the value of the vehicle involved. If you have $100 deductible the premiums can be significantly higher than for $500 deductible. Should the owner have a collision can that owner afford to pay the deductible? Can the owner afford the loss of the entire vehicle? There isn't a recipe for all. Each case has to be handled on merits and affordability and you have to weigh the premiums against the possibility of loss.
As any of my vehicles get older and the value diminishes, when it hits a certain plateau I quit carrying ANY collision coverage. I carried comprehensive coverage mostly to cover total theft and I put a deductible on that, which lowered the premium significantly but makes me totally responsible for the interior spills and rips, and replacement of windshields if necessary, as well as the nit picky types of vandalism. In my case the plateau for any car is $4000. value after which I must weigh junking it or replacing the nit picking things that go wrong from comprehensively covered incidents. On all my cars (3) I carry $500 deductible collision and comprehensive coverage until I reach my personal plateau for value. That is acceptable to most banks and financing or leasing companies. You have to apply similar logic to any situation in order to weigh the financial aspect of the matter.
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