Friday, 16 November 2012

The Peril of the First Yellow Bar and Other Things

You know, I think I can summarise the three things that have made the biggest dent in Prius fuel economy as follows:

  • Short journeys under 10 minutes
  • Starting from cold, in cold weather
  • Shoving things on the roof like roof bars and roof boxes

Now, this may seem obvious, but some people may think that a Prius is a special car that can take any kind of load and conditions and still return 60+ mpg. True, the car is fantastic and does return great fuel economy; my bank balance is testament to that. But it is still a machine designed by humans and is fallible.  You have to treat it within the limits of its capability.

Let's take the first bullet point above.  I'm the main parent taking the kids to and from school.  At 2 miles, the journey is too far to walk for a five and seven-year-old, and so we must needs take the car, at least part ways.  Since this is usually the only trips I make with the car most days it means two short trips with cold engine starts.  This is why I've entitled this post "The Peril of the First Yellow Bar and Other Things".  As you may know, the Prius has a real-time "Consumption"display screen that you can see on the LCD panel.  It is a bar chart segmented into 5 minute marks along the x-axis.  Ninety-nine percent of the time I get a low mpg figure for the first 5 minute segment of the journey - the first yellow bar.  (You can see a photo of such a screen on this link on Flickr.)  It usually hovers around the 25mpg mark in this segment.  Ouch!  A yellow peril indeed   If you ask the Prius to do lots of journeys like this then your overall fuel economy will suffer.  Only when doing regular journeys of, say, longer than 15 minutes, will you start to see overall improvement.

Yes, in my opinion, the temperature plays a part too.  In cold weather the engine has to burn more fuel to reach operating temperature.  If you are lucky enough to have a garage to shelter your car then I recommend it, especially over the winter months.  Your engine will reach operating temperature quicker.  The Prius, just like conventional cars, has to burn petrol/gasoline to reach this temperature.  The electric motor plays no part in this.

There is a body of opinion that thinks that sub-30mph speeds don't really adversely impact fuel consumption.  I disagree.  Even at lower speeds my roof bars create aerodynamic drag - a resistive force that wants to pull the car down to a slower speed.  This drag certainly increases in magnitude the faster I drive.  Nonetheless, it is always present and always gnawing away at my fuel efficiency. I do stuff that often require me to have them on.  I do take them off as often as is practicable.  Experience, and the data I gather, has shown this to be prudent.

I hope I haven't scared you off by this post.  That is not my intention.  I love my Prius and the efficiency I enjoy with it. However, I just wanted to show that it is a humble machine at the end of the day that deserves to be treated well in order to get the most out of its fantastic hybrid technology.


  1. Hi Sean,
    I've been looking around at Prius's for a bit now and wondering if I should make the jump to a mk2 or mk3 from my small Fiat Panda(see my blog).I'm getting 43 average but we are using it more and more and as we have 2 small kids I would like something bigger and a prius sounds great as it would also be cheaper to run than my Panda as well as a lot safer. Are you still finding it a great car and how is it in the snow etc? I wouldn't mind taking it to Italy and Spain this year to see family so I think it would be perfect.. good work on the blog

  2. Hi Mike - so sorry to take so long to get back to you. To address your questions, yes I am still finding it a great car, mine being a 2nd gen - or mk.2 - Prius. For a 6-year-old car it gives me no trouble at all. It is £10 per year for UK road fund licence. It is also cheap to insure. Yes it is safer - 5 stars in the Euro NCAP tests. The snow question - with any car in the snow it is mainly about grip. To drive the car forward the tyres need grip, whether with 2 or 4 wheel drive. Stopping, you need grip too, naturally, and you stop with the same number of wheels whether 2 or 4 wheel drive. So instead of worrying about 2 or 4 wheel drive you need to think in terms of grip and that means swapping to good winter tyres. We have winter tyres on on our old Volvo S40 - a 2 wheel drive car - and it is noticeable. I have standard tyres on the Prius. To move off I sometimes use EV Only mode to reduce torque applied to the wheels then switch back as soon as I build up a few mph. So, if you're thinking of a Prius then concentrate on other factors and budget for a good set of winter tyres. The only area I would raise is the luggage capacity. I have added Thule roof rails (see my posts) and put on a 360 litre roof box. This has given me the extra space needed and as you may have seen from my photos I have carried substantial loads with the Prius. Prius legroom is great all round and the kids' feet shouldn't kick you in the back. I say 'go for it'. Thanks.

  3. Hello Sean -
    I am a brand new, excited enthusiastic Prius owner of a previously owned 2007 model. It came to me with 63K or so miles on it, so I look forward to many years of great mpg and hybrid driving. I found your blog online and have gone through the entire history reading every post. It is so helpful and informative, I just wanted to say thanks. I am excited to try your fix for the key fob, as I think the little plunger thing may be misaligned on one of my smart keys. Anyway, I look forward to seeing more of your mpg posts as well as tracking my own. Also enjoyed the jump start tutorial - good to know as a "just in case". Anyway, thank you again, and I'm so excited to be a Prius fan and owner! Take care -
    Mary in Northern California, USA

  4. Hi Mary and thanks very much for your comment. In fact, it is comments like these that make me realise how worthwhile it is running a blog like this. Thank you. I have a few 'mpg updates' to make to the Prius MPG page so look out for these soon. In the meantime, enjoy your Prius!