What's a fair way to share fees in a group road trip with a personal and a rental car?

Here's how I would go about dividing the expenses:

1. Tally up the expenses. This should be the total of rental fees, fuel, tolls, and an agreed-upon amount for amortization & insurance for the owned car. Everyone, including the owner of the car (let's call her Jenny), must contribute 1/12 of this total amount.

2. Each person then tallies up the amount they have outlaid during the trip. For example, if Alex pays for the rental car, that counts as an outlay for him. Importantly, Jenny's outlays should include the agreed-upon amount for the amortization & insurance; this is effectively an in-kind contribution she has made to the total expenses for the trip.

3. If a person's total outlays are less than 1/12 the total cost of the trip from Step #1, they owe money to someone else in the group. If a person has paid more than 1/12 the total cost of the group, someone owes them money. Since (by definition) the total expenses for the trip are equal to the total outlays, the total money owed should equal the total overpayments.

The only tricky part is figuring Jenny's in-kind contribution to account for the cost of using her car. In the USA, one way to do this would be to use the IRS Standard Mileage Rate, which is effectively a nationwide average of the per-mile costs of owning a vehicle. Note, however, that this standard rate includes fuel costs; so if you go this route, Jenny should pay for all the fuel for her car.

In other countries, similar standard "lumped rates" may exist, and may be quite different depending on the local costs for owning a vehicle and what is included in the rate. Consult your local revenue agency.

Here is a way to split the costs equally:

The car owner pays:

(cost of the car only/number of passengers in the car)

Everyone else pays:

(((cost of the rental + the car) - (cost of the car only/number of passengers in the car)) / (number of passengers in total - 1)

In this way:

• The owner pays a split cost of the car only
• Everyone else pays a split cost of everything combined - what the owner paid

Sum the cost of the van rental and the fuel; divide the total by eleven. All except the car owner pays. The car owner gets a 'free' trip but bears the burden of driving and the fixed costs of maintenance and repairs, insurance, etc. Simple and, I believe, many would consider it to be a fair split of the costs.

Here's the simplest approach:

• owner pays nothing
• everyone else pay all running costs equally (note: owner brings car to trip with full tank, and at end of trip it's filled up as part of the trip's cost)

Why?

• it's simple! People become annoyed with complicated calculations
• it's fair enough

Why is it fair? Think about all the "hidden" costs the owner incurrs:

• risk of damage
• owner carries responsibility of paying for any mechanical problems that occur during the trip
• devaluation due to large number of extra miles (it's a road trip after all)
• extra maintenance due to conditions (eg cost of anti-freeze in winter)
• extra wear and tear (extra tire wear, brake pads, etc)
• general maintenance (scheduled servicing coming up earlier than would otherwise)

I did the numbers once and "owner pays nothing" is close enough to fair; the "hidden" costs (s)he incurrs is about the same as the total hard costs divided into the other travellers. The more travellers there are, the greater the owner's relative contribution would be with this approach.