Tuesday, December 1, 2009

Fuel efficiency insights

I watched a Top Gear episode where Jeremy Clarkson raced a Toyota Prius and a BMW M3 around their test track for 10 laps. It wasn’t a race really. The BMW only had to follow the Prius as it drove the tack as fast as possible.

And what happened? The Prius, with its 1.3L engine used more fuel than the M3 with its 4L V8!

To make matters worse for the pro-hybrid lobby, Clarkson also drove a 1.7 tonne V8 Jaguar XJ6 from Basel in Switzerland to Blackpool in the UK on one tank of fuel – a similar result to the little VW Polo.

So what is going on here with fuel efficiency?

My gut instinct is that weight is the major factor determining fuel consumption. At almost 2 tonnes, the Jaguar seems very impressive. Maybe it’s the weight to torque ratio that is important, as when travelling at a constant speed, power is not so important.

To test this hypothesis I wanted to gather data on vehicle weights, fuel type, engine capacity, number of cylinders, torque, power, and add some dummy variable such as turbo/supercharger, hybrid, 4x4. Then run some regressions with highway mileage and city mileage as independent variables. I found most of the data here, but don’t have the time to add vehicle weights, drivetrain and turbo details into the spreadsheet. When I get a response from the geenvehicle guide with their complete data set I will let you know my findings.

But what does it all mean?  I take away a number of things.

1.  Some of the gains in vehicle fuel efficiency over the past few years may have been offset by and increase in highway speeds and acceleration (more stop-starting).
2.  Don’t scoff at the environmental ignorance of people with V8s
3.  If you buy a hybrid and race it you are just as crazy as Jeremy Clarkson
4.  If you have big car, you can drive slower and save money on fuel
5.  Doing your part for the environment is not as simple as it’s made out by green lobby groups


  1. Cameron - it's not that simple. As an ex car enthusiast (not yet over the love affair) I know that each motor has its own torque curve, and each vehicle it's own drag co-efficient. Combine that with different gearing ratios, and you will discover that every vehicle is built to be the most efficient when driven under certain conditions and speeds, therefore whilst the "top gear" results do surprise, they are not absolutely out of the question, but that dosen't alter the fact that the results were achieved by going well beyond the normal driving range of the Prius, but at a more comfortable gait for the other two cars who are built for fast highway cruising. That is where they are most efficient, and where the Prius loses efficiency.

    I have owned a sports car with a 5Lt V8, a large luxury car with a 7Lt V8, and a toyota Prius. The Prius uses in one month about what the other two each used in one week.

    Tell Jeremy to use a bicycle next time.

  2. I agree, a bicycle race would be great - and they have done that before.

    The main point here is that energy efficiency it is not as simple as it appears at first glance, and we should hesistate before taking the green moral high ground on the issue.

    This is especially the case if we consider rebound effects (my area of research) http://en.wikipedia.org/wiki/Rebound_effect_(conservation)

  3. My concern with hybrids has been that it seems like a scam more than anything designed to keep the public thinking that we have not yet developed all electric car technology yet and the best we can do in the meantime is the hybrid. The hybrid is not designed to show us the future it is design to hold us back from it.

    But back to your subject. Sure it highlights that energy efficiency is not as simple as one might think but the experiment from Top Gear sounds like the results were somewhat predictable considering the purposes of each car. Sounds like a segment to keep the petrol heads in the audience happy. Like putting an Apple Computer up against a tricked out PC on the topic of playing games. The PC will come out on top cause the Apple was designed for something else.