A trip

Three questions before a drive, and they are usually asked together: what will the fuel cost, how long will it take, and what does everyone owe. Stating the car and the route once, at the top, means changing either updates all three.

:distance = 300 miles
:economy = 35 mpg imperial
cost to drive distance at economy at £1.50/litre // £58.45
distance at 60 mph // 5.00 h
split prev between 3 // 1.67 h each

What each line is doing

:distance = 300 miles names a value. Everything below refers to it by name, so a change at the top reaches every line that uses it. See variables.

35 mpg imperial is fuel economy: how far the car goes on a given amount of fuel. This is a British car priced in pounds per litre, so its figure is miles per imperial gallon, the gallon UK brochures quote. A bare 35 mpg is miles per US gallon, a smaller gallon, and would put the bill at £48.67, about a sixth short. 7 l/100km says the same thing the other way round, as an amount of fuel for a fixed distance, and either works here. See fuel economy.

cost to drive ... at ... at £1.50/litre is the fuel bill. The price carries its own volume, so a pump quoting gallons works with a distance in kilometres, which is the ordinary state of affairs in a hire car abroad. See travel.

distance at 60 mph is drive time, which is ordinary unit arithmetic rather than anything the travel forms add.

split prev between 3 divides the line above. Here that is the drive time, which answers who takes which shift; point it at the cost line instead and it answers who owes what. See splitting a bill.

The cost per person

Splitting the money rather than the hours is the same form pointed one line higher, which is easier to see written out:

:fuel = cost to drive 300 miles at 35 mpg imperial at £1.50/litre
fuel // £58.45
split fuel between 3 // £19.48 each

Why the pump price is on the line

Nothing here reaches the network for a fuel price. A pump price is local and changes daily, so the engine asks for it rather than guessing, and a figure you typed is one you can check. The same reasoning applies to the car: 35 mpg imperial is what your car actually does, which is not what its brochure said.

A different car, a different country

Because the car and the route are named at the top, comparing two cars is a matter of changing one line. The economy can be written either way round, and the pump price in whatever unit the pump used. The last line is an American car at an American pump, so its 35 mpg is the US gallon the pump is priced in:

:distance = 500 km
:economy = 7 l/100km
cost to drive distance at economy at £1.50/litre // £52.50
cost to drive distance at 35 mpg at $4.20/gallon // $37.28