Drivers in New Zealand are facing higher pump prices and wondering if they should fill up early or wait until the tank is nearly empty. With forecasts suggesting further increases, the best approach depends on whether fuel costs are rising or falling, according to recent analysis of vehicle efficiency and fuel weight.
Weighing the cost of carrying fuel
Petrol weighs about 0.75 kilograms per litre, while diesel is closer to 0.84 kilograms per litre. A full tank can add anywhere from 25 kilograms in a small hatchback to more than 60 kilograms in a large diesel utility. While that extra weight sounds significant, the impact on fuel consumption is minimal, usually reducing efficiency by one percent or less. This translates to only a few cents per 100 kilometres in most cases.
By contrast, even a small increase in pump prices can cost several dollars per tank. When prices are trending upward, waiting until the tank is nearly empty often means paying more overall. The strategy of filling up earlier becomes cheaper than trying to keep the vehicle light, even though it involves carrying extra weight for a short period.
Carrying fuel is heavy, but it is not heavy enough to offset the cost of a higher fuel price at the pump. For many drivers, the savings from buying earlier outweigh the minor fuel efficiency loss from the additional weight. This makes the decision to keep a higher fuel load more logical during periods of price increases, even if the math seems counterintuitive at first glance.
Strategy for small cars
The Suzuki Swift benefits from small, frequent top-ups because of its relatively small 37-litre tank and fuel use of roughly 6.0L/100km. The most cost-effective approach is to refill when the gauge drops to around half a tank, typically adding 15 to 18 litres. This keeps the average fuel price lower if costs rise while avoiding unnecessary weight.
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Strategy for mid-size SUVs
The Toyota RAV4 sits in a middle category where timing becomes more important. With a 55-litre tank and consumption around 8.0L/100km, waiting until empty means buying a large amount of fuel at the current price. The cheapest approach is to follow a half-tank rule, refilling when the gauge drops just below halfway, typically adding around 25 litres.
For most owners, this means refuelling every 500 to 650 kilometres, or roughly every 7 to 10 days. Unlike the Swift, filling the RAV4 completely when prices look likely to jump can also make sense, particularly before long trips. This method spreads purchases over time and reduces exposure to sudden price increases.
Strategy for large utilities
Large diesel tanks magnify the advantage of buying earlier. An 80-litre Ford Ranger tank can require a very expensive fill if allowed to run low. The most cost-effective strategy is to refill between half and five-eighths of a tank, rather than waiting until the warning light comes on. This typically means adding around 30 to 35 litres at a time.
Because the Ranger has higher consumption, usually around 11.5L/100km in mixed driving, refuelling every 400 to 600 kilometres prevents the expensive $220+ fill that comes with running near empty. While this requires more frequent stops, it keeps fuel costs manageable and avoids the shock of paying high prices for a large purchase.
For towing, long-distance travel, or when a noticeable price jump is expected, filling the tank completely still makes sense. The key is to adapt the fill-up frequency to the current price trend and the vehicle’s fuel requirements.
