I'm planning vacations with a group of friends (12 people), and it involves a ~1200km return trip by car. Only one of us owns a suitable car (4 pax), so we've rented a minivan to transport the other 8, and we're debating on how best to share the costs.
Normally, if none of the cars were rentals, each car owner would just divide the price of fuel and tolls over their passengers, themselves included.
Logically, we could do the same with the rental fees. But as a passenger who could either be in the personal car or in the rented minivan, their share of the cost will be vastly different depending on which car they end up in, for the same trip. That would be unfair to the passengers of the rental car.
We could also share the sum of all fees of both cars across all of us. But that would be unfair to the car owner, who ends up paying a higher trip cost than if it were just his car and passengers sharing the cost, despite owning a car and enduring the associated hassles and yearly expenses.
If we calculate it that way, we need to include the full, actual cost to him of using his car for the trip, including maintenance, amortization, and insurance.
What would be the best way to share these costs?
EDIT to avoid opinion-based interpersonal advice: I'm looking for the most "scientifically fair" solution, some kind of calculation model. Answers that challenge whether we need to be that precisely fair in a group of friends are absolutely right, and all parties have indeed agreed to a "simple and imperfect" solution. We're left with the academical question that is the object of my post: "but what would be the fairest model?"