Suddenly, my fuel consumption is much, much better…

…and I don’t know why!?!


  1. If a Prius is not used for several months, it may become less fuel efficient.
  2. Using the Prius regularly prevents degradation of HV battery.
  3. After a period of inactivity, regular use can restore normal service and fuel consumption.


I’ve been following the great advice on the PriusChat forum to improve my fuel consumption (FC)/mileage. The ADR81/02 fuel consumption figure for the NVW30 Prius is 3.9l/100km (72.4 mpg(UK) 60.6 mpg(US) 25.6km/l).  I have been getting nowhere near that!  Measuring FC tank by tank since I got the car has averaged 4.87l/100km (58/48.5/20.5). I seemed to be getting good results recently as my last 2 tanks averaged 4.7l/100km (60/50/21.3).

But then, suddenly, I seemed to step into the rarefied world of the hypermiler.

The other background bit before I get to the point

I just started a new job and I’m driving everyday instead of taking the bus. For the first time since I bought it the car is being used everyday instead of every weekend.  I used the Hybrid System Indicator (HSI) to measure the distance, FC and average speed each morning.  I then returned home and checked the total for the day.  Note: The HSI is 6.5% optimistic.  You’ll notice that the trips aren’t identical but the conditions are comparable.  Here’s what I measured:

Date Trip Distance Cons +6.5% Ave km/h
5/06/2012 Home to AIS 24.9 4.9 5.2 41
5/06/2012 Round 38.4 4.8 5.1 30
6/06/2012 Home to AIS 12.2 4.7 5.0 36
6/06/2012 Round 24.3 4.7 5.0 39
7/06/2012 Home to AIS 12.2 4.6 4.9 37
7/06/2012 Round 28.4 4.9 5.2 35
8/06/2012 Home to AIS 12.2 4.9 5.2 41

These numbers are in line with my previous efforts.  The trips are short, the mornings are very cold so the engine runs much longer to warm up and doesn’t drive the wheels until abut 1 minute after starting, there’s stop-start traffic followed by a 90km/h zone I have to use the A/C and defogger a little… all of which are bad for FC.
But then suddenly on Friday afternoon and then again Saturday I was scoring 4.0l/100km with little effort and no change of technique or conditions!

Date Trip Distance Cons +6.5% Ave km/h
8/06/2012 Home to AIS 12.2 4.9 5.2 41
8/06/2012 Round 60.7 4.2 4.5 39
9/06/2012 Everywhere 115.2 4.0 4.3 42

I repeated the line for Friday to show that I travelled 12.2km @ 4.9 to get the work, but then only @ 4.0 for the next trip of 48.5km, despite extra stops, hills and more stuff in the car to get a final score of 4.2l/100km.
And today after a very cold start I drove all over and up and down Canberra, made about 10 stops, short trips, several high speed moments and a few hill climbs and only used 4.0l/100km (4.3 corrected). Overall, not ideal hypermiling conditions but still returning much better fuel consumption than when I tried really hard to be good.


  • Built May 2009, Gen III NVW30 RHD Australia-delivered, Base model (like USA Prius Two but with fog lamps as standard.)
  • From July 2009 to June 2011 it travelled 68027km (42200 miles)
  • From June 2011 until I bought it in January 2012, it travelled just 314km (195 miles), sitting at the dealer in Canberra.
  • Now that I look at the service record (!)… the previous fleet owner seems to have missed the 30000km and 50000km services.
  • I use RON 95. I don’t think that there are any winter additives.


  • Canberra is 600m (c. 2000ft) above sea level and it is not flat.
  • Winter (now) averages below 0°c at night and 10-15°c dry with generally sunny days. May averaged -0.2°c at night and reached -5° a few times with 15.6°c days. So far June is averaging 1.0°c (but the last 3 nights were -4 – -5°c) and 12.5°c days.
  • Lots of 70km/h (43mph) and 80km/h (50mph) connecting roads and 50km/h (31mph) or 60km/h (37mph) suburban roads.
  • Once a month I drive to Sydney, 300km away and 600m below, with 200km of mountains in between at 110km/h (68mph)

What I did to help, which didn’t seem to make a lot of difference at the time

  • Pulse and Glide
  • Dead 12 volt battery replaced a few months ago
  • Tyre pressure was in the low 30’s. 42/40 psi for the past few months.
  • Replaced some lights with LED.
  • Minimal A/C and defogger use. Heater off when warming the engine, then on Auto 21°c.
  • Headlights off when waiting at traffic lights. (Trick I learnt driving in Japan; doesn’t dazzle other drivers, saves a bit of fuel.)
  • On cold mornings, I let the car idle for 50s before I move.
  • Garmin 2460LT with ecoroute HD shows coolant temperate, RPM etc. and instantaneous fuel consumption. (It is more accurate than the HSI.)
  • Will be buying a DEFA engine heater. Possibly.

What is working against me

  • Ovation ecovision tyres’ rolling resistance is an unknown quantity, except to say that if their poor grip is a anything to go by…
  • Garage stays above 5°c. Coolant temperature is 10-15°c on start in the mornings.
  • I live at the bottom of a short but steep hill, great for returning in EV mode, bad for starting on a cold morning.
  • High average road speeds in Canberra make P&G tricky and EV less useful.

What is my problem?

  • The US EPA standard is 51/48/50 mpg(US) or about 4.6-4.9l/100km, which is what I was getting before. The ADR81/02 figure of 3.7/3.9/3.9 litres/100km is obviously unrealistic. But the UK claims 72.4mpg(UK) & 3.9l/100km.
  • That’s 2 trips. Show me 2 tanks and then you’ll have some evidence.

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