Prius tank-by-tank economy report

When I calculate fuel use as I fill the tank I’ve been averaging 4.9l/100km (57.65 MPG(UK) 48.23MPG(US) 20.41km/l with HSI indicating 4.6-4.7l/100km).  This is not as good as I had hoped.  But then I replaced the dead 12 volt battery and pumped the tyres up to 42/40psi from the low 30’s that they had been at.  Expecting far better fuel economy I was disappointed to return 4.8l/100km on my last tank (HSI indicated 4.5l/100km).  A few ideas come to mind:

  • The Prius fuel tank is actually a bag designed to prevent fumes and thereby reduce emissions.  It seems to vary in size every time I fill it.  Despite being a 45 litre tank, the most I have been able to put in is 35.21 litres and that was with a very slow fill.  Maybe I’m not filling it to the same level each time?
  • I let my brother-in-law drive for about 40km, at city and highway speeds, with bursts of full acceleration and braking.  I should have set the trip meter just to see what he used.  A rough calculation suggest that if his trip was at 8l/100km, then that would change is 4.6l/100km tank into 4.8l/100km.
  • The last tank was Shell E10 94RON.  If I’m generous and only allow 3% reduction in economy, then that explains 0.15l/100km of the difference.  (Current tank is BP 95 RON)

The ADR 81/02 test combined figure is 3.9l/100km.

What’s the ADR 81/02 test?

The ADR 81/02 test is run on a dynamometer with account made for the aerodynamic characteristics and weight of the car.  The city cycle runs for 13 minutes and is a stop-start ride with speeds up to 50km/h.  The car spends a total of four minutes stopped during the 13-minute cycle.

The extra-urban cycle involves the car accelerating from a standstill and holding various speeds up to 120km/h.  for some reason the car is stopped for 40s of the 6 minute 40 seconds extra-urban test cycle. has more details, but you need to know your maths and stats to make sense of it all.  I’m still trying to determine if a Plug-in Hybrid is tested using EV mode or if it is forced somehow to behave like a standard hybrid.  Read this and tell me, if you could.

Does the ADR 81/02 test have any basis in reality?

Most fuel economy tests employ conditions that could be considered unrealistic.  For a start, when you drive somewhere, you don’t stay in a shed on a rolling road at a constant temperature; you move around going up and down hills, with and against the wind and on varying road surfaces.  However, the tests are reliable in the sense of being repeatable and comparable across a wide range of vehicles.

Now, on this planet…

In the real world, there are many more variables.  It is difficult to compare the consumption of one tank of petrol to the next unless every day is the same journey in damn near identical conditions.  Driving style can account for 30-40% of fuel economy, usually for the worse.  Very cold temperatures can affect economy as it takes longer to warm the engine and fuels sometimes have additives that keep the fuel stable but not as potent.

The lowest figures for the Prius are around 30 MPG(US) 7.88l/100km 25.1 MPG(UK) 12.74km/l in extreme circumstances such as sub-zero temperatures and short journeys or holiday trips with a full load and roof racks.  There was a journalist who flogged a Prius (previous NVW20 model with the 1.5 litre engine) and got about 30MPG(US) despite driving like maniac.

Next steps?

I have to work on my Pulse and Glide technique.  I already found that my pulsing is not as high and my gliding is not as far as I could do it, traffic permitting.

I also realised that a fast uphill trip on Majura road used a lot more fuel than a fast downhill trip with a return via the city.  Maintaining 90km/h on a rough road while climbing 130m and descending 60m might not be as fuel efficient as travelling 60-80km/h with more gradual climbs; traffic lights notwithstanding.

I’m tempted to try some more extreme measures if this tank of BP 95 RON doesn’t perform as I hope.


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