Mileage getting better, but why now all of a sudden?

If a Prius is not used for several months, does it become less fuel efficient? Can using the Prius everyday restore normal service and fuel consumption? I'm beginning to think that something in the car (the HV battery?) has been reset to normal, efficient functioning.

I've been following the great advice on PC to improve my fuel consumption/mileage. But I'm nowhere near the ADR81/02 figure of 3.9l/100km (72.4 mpg(UK) 60.6 mpg(US) 25.6km/l). Tanks have averaged 4.87l/100km (58/48.5/20.5). My last 2 averaged 4.7l/100km (60/50/21.3), so I thought that I was making slow progress. That is until my last few trips.
I've 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. This week I reset the HSI each morning, took a reading when I got to work and then again upon returning home. The trips aren't identical but the conditions are comparable. (The 6.5% is the HSI error.)
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
Suddenly on Friday afternoon and then again today (Saturday) I'm scoring 4.0l/100km with little effort!
8/06/2012 Round 60.7 4.2 4.5 39
9/06/2012 Everywhere 115.2 4.0 4.3 42
On Friday, 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.
Car

• 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.

Environment

• 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 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.

 

On the weekend my trips averaged 4.7L-5.0L (corrected 5.0L-5.3L 46mpg(US)) under some tough conditions. Back to work after the long weekend and again I'm scoring 4.0-4.2l/100km (corrected 4.2-4.5L 54mpg(US)).
It looks like I am able to use much more of the HV battery. I don't seem to changed my P&G technique at all; however, it does seem that I can glide much longer than before. Obviously my trips aren't consistent length and I'm not recording all conditions, so I might be missing a confounding variable.
Maybe I wasn't getting into Stage 4 previously and this was affecting my glide?

 

I did mention it, but as a bullet point: Canberra is in Winter and is getting colder, so I'm expecting worse FC. I don't think that fuel is winterised here but everything else is. The FC is 0.7gph to charge the HV when stationary? My colleague's Ford Mondeo TDCi (diesel) displays 1l/h when it is idling, so 2.65l/h is quite a drink when you think you're just idling!

I have a Garmin ecoroute HD and I've set the gauges to RPM (main gauge), coolant temp, speed, intake air temp, engine load. As I drove home today I realised that Ignition Timing is more important than a fourth speedo (HUD, dash, Garmin GPS map display) so I swapped the gauge. I think that throttle position is an option too.
I've been waiting 50 seconds before driving if the engine is cold, but which stage it is 25°c - 77F or so. (The HSI consumption is 99.9l/100km as I move off, which is Bugatti Veyron territory!) I generally reach 40°c - 104F by the top of the hill outside my home and then the engine will start-stop at the first roundabout. So I wonder if that little wait in the morning is saving a lot of fuel and HV?
Tomorrow morning will get down to 0°c - 32F and will be about 7°c - 45F in the garage.
What coolant temperature/ignition timing values would indicate moving through the operational stages?

Back to the main topic, I've narrowed the reasons down to three possibilities:

1. My Pulse and Glide technique has improved. Subtly, but I can glide noticeably further than before.
2. I'm waiting 50s for the cold engine to warm before moving off and then I move gently to not drain the HV battery.
3. The HV battery went stale from 6 months of under use while the car was one the dealer's lot and from recent daily use I've resurrected it to its former glory. (I have no evidence whatsoever and no-one has suggested that such a thing could happen.)

 

Srivenkat, I do use the parking brake every time. The car is garaged and it has been dry recently, so the brake pads never get stuck on. I would hear and notice sticking brake pads.

BTW, petrol is not winterised in Australia. Diesel is winterised in Canberra and alpine areas.

I think that I am just more patient with the car. I pre-warm the engine before starting, I've blocked the grill, I wait until the timing goes positive before asking anything of the car, I delay the heater until there's some warmth in the engine.
So I guess I'm saying that the Scangauge-e and Garmin ecoroute HD are saving me petrol.

My FC has reduced 1LHK from when I bought the car and about 0.5LHK since this time last year.
From January to April 2012 I averaged 5.0LHK (47mpg US)
From January to April 2013 I averaged 4.2LHK (56mpg US)

Winter is just starting, but I hope that the techniques I've learnt will help me to maintain good numbers this winter.