Does this tactic at the bowser save you money?

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With fuel prices at historic lows, there’s a real temptation to fill your fuel tank as far as you can. And we all know we can squeeze in just a little more after the auto shut-off activates on the bowser.

Many of us decide to put in just a little more to round the price up, and get as much as we can while prices are low.

But is it a good idea?

The short answer is: no.

When you fill your fuel tank, the air that would be trapped in the tank is vented out through a small tube that runs parallel to the pipe that lets fuel in.

Overfilling the tank means that tube sits below the level of the fuel – that’s how the auto cut-off works because there’s a sudden increase in back pressure that tells the auto shut-off the tank is full.

Regularly overfilling the fuel tank may result in that small tube filling with fuel and, because it’s a small tube, fuel doesn’t drip back into the tank easily. If the breather tube is full of fuel (or anything else), air can’t escape and every time you try to fill the tank, the auto shut-off will activate because the message it’s receiving is that the tank is already full.

You, of course, can read a fuel gauge so you know the tank isn’t full. In this case, you can try filling the tank very slowly (not easy with modern pumps), which will permit the air to come out while the fuel is going in. That’s fiddly and slow. And because the fuel is going in slowly, the auto shut-off may not operate and there’s a chance you will get an overflow, which is bad for your paintwork, and wasteful for your wallet.

There’s another possible outcome of brimming your tank. Fuel coming out of the underground tank is cool. On a very warm day, it is possible that the fuel in your tank will expand as it warms and may then leak out of the fuel filler.

So, the best advice is to stop filling your tank when the auto shut-off activates.

There is an exception: some cars have a bit of a kink in the filler line, and the back pressure this creates can also fool the auto shut-off into thinking the tank is full. There isn’t much you can do about this, but if your car is one with this unfortunate quirk, you’ll soon learn how to recognise it. The best way around this problem is to fill the tank more slowly or pause occasionally to let the fuel make its way down the filler neck.

Do you try to squeeze a little more into the tank after the auto shut-off? Will you stop doing that now?

This article first appeared on

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Written by wordsmith


Total Comments: 9
  1. 0

    I always bring it up to the next 0.25 litre to achieve a nice rounded fuel consumption figure.

  2. 0

    I always stop when the pump stops. I read somewhere that to keep filling can cause overflow.

  3. 0

    A better cost saving trick is to buy your fuel on a cold day. Fuel expands with heat so on a hot day the fuel expands and the pumps measure the volume being pumped. On a cold day, or early morning, therefore you will get more fuel than you will on a hot day for the same price. Probably not a huge saving but every little helps..

  4. 0

    I doubt that you would get much fuel in your tank if you tried to fill up at your browser. You might get some if your filled up at a bowser.

  5. 0

    What, is this suppose to be a real article, for 40 years I have topped-up the tank on most occasions in numerous different cars and have never had an issue.

    My current car (3 years now) uses more fuel than any other car I’ve had and with the prices now and the fact it needs 95 octane I have been filling it when the price is low until I can see the fuel in the filler neck. Again no issues.

    And this crap about it expanding in the tank, that’s true, but by the time you have driven the car a little way you would have used more fuel to reduce the level than it would have expanded to overflowing.

  6. 0

    With fuel prices bouncing 20 to 40 cents a litre I keep it filled, with two tanks, I fill when the down to half on either tank. NEVER had ANY OF the ISSUES listed in this article in 57 years driving, filling dozens of different model cars, story is a load of CRAP. Some cars you can fill until overflowing others you need to stop when shut off.

  7. 0

    Keep yourselves nice chaps. Great to offer other opinions but nice to keep it in the spirit !



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