Filling up a rental car overseas is usually simple once you know a few local conventions.
Most countries outside North America sell fuel by the liter rather than the gallon, and prices are often higher than US averages, particularly across Europe. Double-check whether your rental takes petrol (gasoline) or diesel before you fill up — the nozzles are usually a different size, but it's still one of the most common rental mistakes.
In many countries, especially across parts of Europe and Asia, full-service stations are common, meaning an attendant pumps the fuel for you and a small tip is customary. Self-service stations increasingly accept card payment directly at the pump, though some rural stations remain cash-only.
Return the car with the same fuel level you received it at, and keep the receipt — rental agencies often charge a steep refueling fee if the tank isn't topped up as agreed.