Factors affecting fuel economy

Introduction

The fuel economy figures published by car manufacturers are obtained in ideal conditions: cars running in a laboratory on rollers, highly trained driver, etc (see [1]).  Such idealized figures are useful for comparing the fuel economy of various vehicle models, but they are not always easy to reproduce in our day to day driving.  The problem is that fuel economy seems to depend on many factors, and those conditions are not necessarily taken into account in the idealized tests.

My personal experience is that the fuel economy that can be obtained with a car varies a lot, as it may depend on several factors: temperature, traffic, road topography, speed, etc.  Although the fuel economy obtained with the Prius does not vary as much, it still shows significant variance.  Consequently, it is a bit hazardous to try comparing the fuel economy obtained by different Prius drivers across the continent, as they drive in very different conditions, and probably in a very different way. 

My main motivation when I purchased a Prius was to drive a car that can provide the best possible fuel economy.  But in addition to being equipped with high tech devices to save fuel, the Prius offers a nice display board that informs the driver, in real time, on how good (e.g. "ecological") his driving is, and how those results compare with those obtained yesterday or last month.  However, in spite of how advanced the technology might be, it is believed that the way the driver operates the car has a strong influence on fuel economy - and that is independent of the car.  So the idea behind those real time displays is to "put the human into the equation", by providing him with info that will help him achieve better fuel economy.  Thanks to those gadgets, one thing I often try to do is to drive in such a way to get the best possible fuel economy.  The problem is I don't know whether my efforts do have an effect, because the fuel economy figures vary a lot.  It is very hard to make any comparison, as the variance is too high.

In my last post, I studied the problem of the fuel economy gauge accuracy: I recorded the amount of fuel, distance, and fuel economy (FC) every time I filled the tank up.  It revealed a nice seasonal variation, but it was based on average FC between the fill ups.  Considering that I might drive in very different conditions between the fill ups, I needed something more accurate, and more real time.

Therefore, I designed another scientific experiment, which goal was to:

1. Find the various factors affecting fuel economy, and their relative importance.
2. Find out how much the way I drive does affect fuel economy.

Fuel economy gauge accuracy

Introduction

The Prius is well known for being the champion of all hybrids in terms of fuel economy (FE).  It is equiped with nice, real-time gauges that inform the driver about the way he drives.  The system is aimed at providing instantaneous feedback to help him achieve even better fuel economy.  But the Prius is also equipped with a more conventional trip computer, that displays the average fuel economy since the last gauge reset.  That tool is useful for those (like me) who like to monitor the amout of fuel they use on a daily basis.

When the Prius 2010 was introduced, some owners noticed there was a difference between the value displayed by the fuel economy gauge and the actual amount of fuel one could put in the tank [1].  One could expect such a difference, as the amount of fuel measured by the pump at the station may be erroneous.  However, several chatters said that the fuel economy gauge display was consistently more optimistic than in reality.

To better understand the problem and perhaps identify potential causes, I have started a little experiment.