Starting point that finishes a lap with scattered fuel

Cans of fuel are placed at points around a circular racetrack, and together they hold precisely enough fuel for one full lap. A car starts with an empty tank. Show that there is some can from which the car can start and drive the whole lap, collecting fuel as it passes each can.

Start at the point where the running fuel balance is lowest. Imagine driving a lap from anywhere with an unlimited tank allowed to go negative, tracking fuel collected minus fuel used. Since the total is zero, the balance returns to its start; beginning at the location of its minimum makes every subsequent balance non-negative, so the car never runs dry.