We're in the process of planning a trip in a PA28 from the Midlands to Mallorca. Worked out that if we went 4 up, we could only carry 65 litres of juice with a very small baggage allowance. To keep an hour in reserve, we'd have to land and refuel every hour!
What a difference an adult makes - going 3 up allows fuel to tabs and a reasonable amount of baggage.
Hope that helps,
AV8