- 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.