Can You Save Money On Petrol By Half-Filling Your Fuel Tank?

It's often said that driving with a half-full tank will save you money on fuel, but do the numbers really stack up? Tim Trudgian from the Australian National University investigates.

Picture: Newtown Graffiti

Fuel prices may be at historic lows at the moment but when they rise again, what is the best strategy to save money at the bowser?

There are many suggestions for saving money such as when and where to fill up, what time and day of the week and how to predict the highs and lows of the price cycle.

One theory on how to conserve fuel is to put less of it in your tank. The thinking is that you carry around less weight in the car, which means the engine does not have to work as much, which means a saving on fuel.

Does less save more?

But how much of a saving? And is this worth your while? This question has been asked (see here and here as typical examples), but not satisfactorily answered.

To answer this we need to look at the fuel economy of the car. This is often quoted as litres per 100km, say, 6L/100km (typical for a small car running basic unleaded petrol). This means that under some given conditions, on average the car will use 6L of petrol to drive 100km.

The conditions (sometimes stated in the quote, but often not) could be for: city driving, highway driving, two people in the car, etc. 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.

One study by consultants Ricardo Inc examined the effect of extra weight on fuel economy.

Roughly, it found the fuel economy increases by between 1% and 2% for every 100 pounds (43.5 kg) of weight added inside the car. This figure does not factor in any extra weight outside of the car, such as roof racks, trailers or sidecars.

Suppose we took the upper limit, 2%, and assumed a full tank of 60L in a car that is rated at 8L/100km (a typical family car).

Crunching the numbers

Since petrol has a density of roughly 720 grams/L 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 8.16L/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 8.08L/100km: the car is travelling more efficiently than it was with a full tank.

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 L/100km) at the start of our journey.

But how much do we save?

The actual price of fuel does not matter; the question is how much further will we travel in the half-twice scenario? This is not a simple high-school algebra calculation.

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 8L/100km, density of petrol at 720g/L, a loss of 2% of efficiency per 43.5kg, and a 60L tank, we find that we cover roughly 3.5km more in the half-twice scenario: not a lot by anyone's standards.

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 we save a little over five minutes in the half-twice scenario. Again, not very much.

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.

Bigger is better

We can demonstrate a significant saving if we look at larger vehicles. Take a Toyota Landcruiser 70 Troop Carrier GXL: this is a diesel engine with an urban fuel economy of 14.3L/100km.

You make a saving here, not because the car is inherently efficient, but because its fuel tanks are enormous: it comes with two 90L 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. If not, then you will need to find another strategy to save on your fuel costs. Happy hunting.

The Conversation

Tim Trudgian is Research Fellow in Mathematics at Australian National University.

This article was originally published on The Conversation. Read the original article.


Comments

    i only fill up half because it makes me feel a bit better spending $60-70 instead of $130+

    Plus 99.9% of my driving is done around town and not on highway so i probably do go a bit further on a tank.

    More a mental thing i guess.

    Last edited 23/01/15 10:26 am

      $130 to fill the tank? See this is why I drive a small car.

        That's gonna be like a 100L tank and my only guess would be a light commercial vehicle like a delivery truck or similar.. Maybe a four wheel drive? I dunno..

          Its an 80L tank, where i live we pay way higher prices than almost anyone else in the state. I only run 98 octane which is $1.52 a litre at the moment, so slightly under $130 It was great fun when 98 was $1.79 a litre and had to take the car on a long trip! woohoo

          Last edited 23/01/15 12:37 pm

        I recently brought a motorbike and my god $17 to fill a tank that goes 300+ km its insane haha

          Motobike - lower fuel expenses, higher hospital and funeral expenses. See, it all balances out.

            Lower instances of depression though, so Therapy costs less having a bike ;-)

      City driving as in stop start traffic you're more often you're going to have a higher litre per kilometre usage of fuel, where as highway driving when you're staying at a constant speed for most of the journey you're definitely going to be using less fuel per kilometre than city driving.. Our car uses somewhere around 11L per 100KM in a city environment but on the high way we're looking at around 7L per 100km..

    Unfortunately where I've recently moved to is a greater distance to any petrol stations and I don't pass a single one in my daily trip to work. So for me any saving would be negated with the travel distance and time to get to a servo. But it's good to have the science back you up.

      My thoughts exactly on my normal work run any petrol station is a detour.

    We have also been told in the past that we shouldn't drive until we have almost nothing left as the fuel pump may end up sucking up residue that has collected at the bottom of the tank. I don't think this is as common on newer cars but could be an issue if you do lots of gravel driving.

    Personally, I try and fill the tank whenever it gets under the half-way point.

      When my fuel pump died on my 2007 model car, my mechanic recommended not letting the tank get down too low as it can lead to fuel pump damage. So while a 2% reduction in fuel usage may be great, it gets cancelled out pretty quickly if you have to drop a whole heap of cash on a new fuel pump.

      yeah not common on newer cars, Actually i'd say this would be more suited to car's pre 90 or mid 80's even most fuel pumps have built in filters.

      How would the fuel pump "suck up" any residue? It outputs fuel not sucks.

        He means the fuel pump in your car..... If there isn't much fuel it can suck in all the decry and junk in unclean fuel

    Currently I fill up the tank since it's cheap.

    Check with your insurance company for perks. Ours offers Woolworths gift cards at 5% off the actual value. We get all our petrol at 5% off, plus 4c off per liter.

      RAA in South Australia has this too if you're a member. Right now LPG is about 62cpl. I can buy a $20 prepaid recharge for my phone in the petrol station and get 10cpl off my fuel - as well as the 5% discount from the WISH card. Equates to over 20% discount without having to purchase any thing that I wouldn't need to anyway.

    Have you considered the time benefit when using high flow diesel pumps? You'd be saving even less time doing a half fill.

    They say you get an extra 3.5km and also save 5 minutes of time in the half tank scenario. But if you have to drive out of your way to get petrol then you lose this advantage straight away. I fill up because it's more convenient and it means less time messing around getting petrol. It also means I can take advantage of lower prices and have enough petrol to get me through periods when petrol is unusually high. If petrol prices are average or unlikely to go up then I don't bother filling up again until the tank is lowish. I also like the convenience of having a full tank when I need it.

      Thats what i was going to say:
      1) you'll spend more time filling up half way twice (pulling into station, and paying, waiting to get out if the road is busy)
      2) lose some of that 3.5k if you have to go out of your way to fill up
      3) if your forced to fill up on a more expensive day, it could add upto 20c a litre (seen it jump that much between morning and night on pricy days), which if you fill up 25L, is $5, which is worth 5L atm (with discount) which will get you considerably further than 3.5k, whatsmore at current discount prices 3.5k costs 21cents in petrol, so if by half filling you had to pay 1cent more per litre, 25L will cost you more than you save.

    One thing also to consider in this scenario is that the lower the amount of fuel in your tank the more likely you will suck up particulates etc in the bottom which can lead to higher maintenance costs due to fuel filter getting clogged and follow on effects.

    That's why you should just buy a gas car (: less then $50 to fill a tank!!!

    I am amazed by the apparent fluctuation in petrol prices because for me it has not changed for over 10 years. 10 years ago I used to put $50 worth of petrol in my tank and I put $50 worth of petrol in just yesterday. No change simples.

    Am I the only one who does not care too much about fuel prices?

    I drive a car that requires 98 ron fuel/premium, The only time I need to fuel up is when my fuel light just comes on, I don't do the whole "Only buy fuel on x day because it's 4c cheaper"
    I find it funny that you notice on these "cheap" days are massive lines of car spilling out onto the road, probably with a good wait time of 10 - 15 minutes, just so they can save $5 off their fuel bill.

    They probably have that same $5 'savings' sitting in their ash tray. I find my time valuable so I'm not one to want to wait for such awesome savings.

    right now it's about $1.20 for premium which is cool.

    tl;dr - I just get fuel when it's needed.

    Last edited 23/01/15 3:21 pm

      I'm the same I buy it when I need it.

      I've been told by numerous people that you should never wait until your fuel light comes on... in any vehicle... as the ratio in heavy impurities that have been left in your tank for ages then goes through the engine. If you keep at least a quarter - half tank at all time then the impurities should stay at rest in the bottom of the tank...

      Though how you're meant to eventually get rid of them is beyond me, lol...

        Complete nonsense, most modern cars have at least two filtering stages (sometimes more) to remove particulates. Solid matter will not get into the fuel lines if you run it low. Any liquid impurities such as water are generally removed by the mixing and sloshing effect of driving as the fuel is consumed, any water in the fuel as the tank gets low will be minuscule. You will not damage anything at all running a modern car down to almost empty every fill.

        that rule no longer applies to most cars now days, you will also find that most fuel pumps sit at the top of the tank too, basically what Steve f said :)

    Agreed. Just buy the stuff when you need it.

    I'm the same, but then I drive a V6 daily and a V8 weekender, and I don't drive economically. I enjoy driving, and to switch to a small car to save a few bucks at the expense of enjoyment is something I would never do. people that would stress over 3.5km would have to be the people that don't care what they drive, just that they get from a to b with no fun or hassles. Both of my cars get BP ultimate and whenever the needle hits a quarter they get filled to the brim.

    Filled to the brim. Always. Usually I refuel once a week on a Thursday some times it's nearly empty sometimes it's just below half depending on how much additional driving I've done.

    Picking a petrol station isn't an issue seeing as I pass 7 different petrol stations on my journey to and from work and if I see a couple have put their prices up significantly but one hasn't yet then I'll pull in and fill up because by the afternoon they normally all match each other though lately that hasn't been an issue.

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