Fuel is the one running cost on top of your rental price, and it is cheap by world standards. Here is what matters in practice.
Which petrol
Petrol stations in Vietnam commonly sell RON95 (often labelled A95) as the standard grade, sometimes with higher-octane options and E5 ethanol-blended petrol at some pumps. Small underbones and scooters — the Honda Wave, Vision, Click, Air Blade, Sirius, and Mio families that dominate Hanoi rentals — are designed around RON95. There is no benefit in paying for higher octane on a small rental bike, and no reason to seek out anything unusual. If a pump only offers an unfamiliar option, ask the attendant; they will guide you to the standard grade every local driver uses.
Where and when to buy
Petrol stations (cây xăng) are everywhere in Hanoi, mostly major-brand forecourts with attendants who fill for you. State the amount in dong or say “đầy” (full). Stations get crowded at rush hour and can be scarce on rural stretches outside the city, so on any day trip out of Hanoi, top up before you leave town rather than gambling on the next station — see best-motorbike-for-day-trips-from-hanoi for planning.
How to use the pump
You stay on the bike or dismount, either is fine. Attendants handle the nozzle; just open your seat or your filler cap. Petrol caps on underbones are often under the seat or on the frame — check during your handover inspection (rental-motorbike-inspection-checklist) so you are not hunting for it on a dark roadside later. Pay cash or QR; most pumps take both.
Agree the fuel rule with the shop
Most Hanoi rentals hand over the bike with some fuel and expect it back with some fuel — exact expectations vary, so ask at handover and note what was agreed. Do not plan to return it empty; stations near your drop-off are easy to find. The general policy picture is in motorbike-rental-fuel-policy-hanoi.
Consumption, roughly
Small automatics and underbones use roughly 1.5–2.5 litres per 100 km depending on load and city stop-start riding. Fuel is inexpensive relative to rental price, so even a full week of commuting costs less than a single day’s rental — see motorbike-rental-price-hanoi for the published rates. Rough figures vary by model and conditions; treat them as planning aids, not specifications.
One habit worth forming
Never let a rental run to empty in traffic. The fuel gauge on an underbone is approximate and reserve taps vary by model; refuel at a quarter tank and you will never learn the details the hard way.