While there are many tips on how best to save your fuel, one question that continuously arises is whether it is better to completely fill your tank or leave it half full.
Those in favour of not completely filling your tank argue that the more weight in the car – the harder the engine has to work to move the car at the same speed – and hence the worse (or higher) the fuel economy.
According to the UK’s RAC service, on average every 50kg will increase your fuel consumption by 2%. It adds that this is based on the percentage of extra weight relative to the vehicle’s weight so it affects smaller vehicles more than larger ones.
Looking at the numbers
Researchers at the Australian National University may be able to provide a definitive answer to this question as they published calculations based on the weight and density of fuel, and how it impacts a car’s economy.
The researchers found that since petrol has a density of roughly 720 grams/litre the weight of the full tank of fuel is about 43.2kg.
So, roughly, your fuel efficiency will drop to 8 x (1 + 0.02 x 43.2/43.5), which is roughly 8.16 litres/100km.
As you drive, you use up some fuel, and hence carry less weight in the car. When you have used half a tank your fuel efficiency is now roughly 8.08 litres/100km: the car is travelling more efficiently than it was with a full tank, the researchers said.
“Suppose that we fill up the tank and see how far we can drive until we run out of fuel. Call this full-once: we have filled the tank with fuel once.
“Consider our filling the tank half-way, driving until we run out of fuel, then filling the tank half-way again, and driving once more until we run out of fuel. Call this half-twice: we have filled the tank half-way, twice.
“We will certainly travel further in the half-twice scenario. Why? We have used the same amount of fuel, but in the full-once scenario, we had to suffer poor fuel efficiency (8.16 litres/100km) at the start of our journey.”
Saving time not fuel
The researchers noted that the actual price of fuel does not matter, but rather the question is how much further you will be able to travel in the half-twice scenario.
“The rate at which we are consuming fuel decreases as the weight of the fuel decreases. We can use some first- or second-year university mathematics and solve a differential equation to find the total distance covered.”
Assuming a fuel efficiency of 8 litre/100km, density of petrol at 720g/litre, a loss of 2% of efficiency per 43.5kg, and a 60 litre tank, the researchers said that we cover roughly 3.5km more in the half-twice scenario.
“This is all well and good, but there is one obvious drawback to the half-twice scenario: we have to make an extra visit the fuel station and this costs us time.
“So, rather than figure out the distance we save in the half-twice scenario, we should figure out the time we save.”
“Suppose we were driving at an average of 40kmh – this is lower than the urban speed limit owing to traffic congestion, slowing down, stopping at lights etc.”
Under the same assumptions as above (that gave us the extra 3.5km), this means you will save a little over five minutes in the half-twice scenario, the researchers said.
“The situation changes slightly for diesel cars. A tank of diesel weighs more than a tank of petrol (and so the savings should be increased), but according to the Ricardo study, the affect of weight on fuel economy is less pronounced.
“Diesel engines are also more efficient, but even with small fuel efficient diesel cars, such as those suggested by the Royal Automobile Club of Western Australia, you cannot save more than five minutes.”
More noticeable with bigger cars
The researchers said that this was more noticeable with larger cars.
“Take a Toyota Landcruiser 70 Troop Carrier GXL: this is a diesel engine with an urban fuel economy of 14.3 litres/100km.
“You make a saving here, not because the car is inherently efficient, but because its fuel tanks are enormous: it comes with two 90 litre tanks.
“Filling both tanks means an awful lot more fuel is carried around, and hence a more pronounced saving. Assuming an average speed of 40kmh, you would save nearly 14 minutes by filling up half as much, twice as often.
If time is important to you then you can achieve savings of between five and 14 minutes – depending on the size of your vehicle – by only filling your tank to the half-full mark, the researchers said. If not, then you will need to find another strategy to save on your fuel costs, they said.