Fuel Gauges and Comsumption for VW Golf

Running On Empty

I have long suspected that fuel gauges are not accurately graduated in general, so I decided to record the rate of decline of my Golf's fuel gauge over a period of a few weeks. These are the results of my little experiment.

I filled my fuel tank with Statoil Unleaded Regular (95 Octane). A VW Golf MkV has a fuel tank capacity of 55 litres and I stopped filling when the fuel pump first clicked, so I have reason to believe that 55 litres is what I had in the tank at the start of the experiment.

Crucially, I conducted the same type of driving right through the experiment. I did not take any long, extra-urban trips; I drove only to and from work, 20kms through the city from my house. All my driving was urban driving, on the same route, at the same time of day. Therefore, the rate of fuel consumption would have been constant at any given stage in the experiment.

When the fuel gauge had reached 1/2 way, I had travelled 350kms. Now if the gauge was accurately graduated, one would expect that I would travel another 350kms on the second half tank, giving me a total of 700kms on 55 litres. Equally, one would expect that at 1/4 fuel remaining, I would have travelled an additional 175kms for a total of 525kms.

However, when the fuel gauge read 1/4 full, the odometer read 500kms travelled.

The next point of note was when the fuel warning light came on. According to VW's manual, when the fuel light illuminates, there are 7 litres of fuel remaining. This would mean that at that point, I would have used 48 litres of fuel. The fuel light came on when the odometer read 560kms. (Aside: the light illuminates earlier when driving on an incline, and turns off on a decline, showing that the fuel sensor is located towards the front of the fuel tank.)

According to the fuel gauge, I travelled 350kms on the first 27.5 litres (0.078 l/km), 150kms on the next 13.75 litres (0.091 l/km), and just 60kms on the following 6.75 litres (0.112 l/km).
According to the fuel gauge, my fuel consumption rose as the tank emptied. As we know this to be untrue (if anything fuel consumption should have fallen as the car got lighter), we can conclude that the fuel gauge is inaccurate. If you like, the top half of the tank is 20% larger than the bottom half.

As for average fuel consumption, I travelled 560kms on 48 litres of fuel. This gives a fuel consumption rate of 0.085 litres per kilometre, or 33 mpg. Contrast this to the official VW fuel consumption figures for urban driving of 0.095 l/km or 29.7 mpg. This better than advertised performance can perhaps be accounted for by the few extra-urban stretches to my commute, more than offsetting cold starts (the engine consumes more fuel than normal in the first few kms after being started until the engine reaches normal operating temperature) and inefficiencies in my driving style.

Usefully, this also tells us that when your Golf's fuel warning light comes on, you can still drive around the city for a further 82.35kms before you run out of fuel; Which should be plenty of time to find a service station.

Update

I repeated the test using Shell V-Power, a high performance fuel with a higher than normal octane rating (99 RON) and extra detergents and lubricants. I discovered a couple of interesting things in this second test.

Firstly, the needle doesn't move from full for 100kms. This indicates that there is a considerable quantity of fuel above the fuel sensor when the tank is full. It also explains the apparent difference in fuel consumption between the top and bottom halves of the fuel tank. When the first 100kms is taken out of consideration, the remaining fuel consumption is...

Secondly, using the V-Power fuel I got to 375kms on 1/2 a tank. This compares to 350kms on standard Statoil Unleaded. V-Power gave 7% better fuel consumption while costing only 4% more (€1.20 per litre for V-Power vs €1.15 for standard fuel). Therefore, the more expensive V-Power is actually better value for money.