Mentality behind terminology: MPG vs L/100km

hmm... I don't have strong opinions on this either way, other than getting rid of the damned US unit system ;-)

But consider this problem, which many people cannot solve, or at least intuit wrong: if fuel consumption is proportional to speed, which is more fuel efficient: 80 kph to, 120 kph back; or 100 kph all the way ? Look at it as a person with a unit/distance fuel economy (FE) number vs a person with a distance/fuel FE number.

 

All that tells me is that I too can also get 4.8L/100km if I drive your particular Prius in identical conditions using the same driving technique. If I'm truly lucky and the stars are aligned correctly. It says nothing about my Prius' mileage. If I can't reproduce your mileage using the same model car, how can I make valid comparisons between different model and make of cars?

If I want to get a truly accurate indication of a vehicle's fuel efficiency, I need to determine Specific Fuel Consumption - fuel used against power output. Then I can get a truly representative comparative analysis of a range of vehicles to determine which one is truly more fuel efficient. This is nothing new: the aero engine industry has always used SFC as a true measure of an engine's fuel efficiency.

I think this is a perception problem manifested by the arbitrary units you are using. For Joe and Jane Average, the two situations you mention above are mutually exclusive, and don't take this the wrong way - it's a strawman argument. You either have a 20mpg car or a 45mpg car you want to improve. Your decision will be how much you want to improve this by. If I had a 20mpg car and I can buy another car that gives 10mpg improvement, I'm not really going to be concerned whether another car's 45mpg actual improvement will be less if I apply the same 10mpg increase. The latter doesn't apply to me, I'm alright Jack, I'll take the Yaris, thanks.

Now if you're talking about manufacturers and FE advocacy, surely the goal is to try to show actual improvements by using an absolute measure that will be real world-reproducible, and that would be SFC.

Surely any FE improvement is all good though?

 

Fuel consumption is not proportional to speed. If it were, both cases would be identical. The relationship is non-linear: 80 kph out and 120 kph back takes more fuel than 100 kph all the way.

Tom

 

Size Matters!!!

I want a bigger house, a bigger salary, a bigger bank account......

If I get a better car and become a better driver, I want my score to go up, not down!

With MPG my score goes up, with L/100k my score goes down, how wimpy is that?

 

I don't care which equation is used, as long as the differences are easily perceived.

I remember when flooring was sold by the square yard. All of a sudden, it was being sold by the square foot. Carpeting that was $15 per square yard was now $1.67 per square foot. Cheap!

If they use gallons per 10,000 miles as the article states, the difference will be easily discernable. My fear is that the automakers will use a smaller number in the denominator to make the differences seem very small. If you were looking at two vehicles, one of them used .05 gallons per mile and the other used .045 gallons per mile, the difference looks miniscule. But it is in fact a 10% difference. I think many consumers would be fooled by this.

Just my 2 cents.

 

I see what you're getting at but really all measures of efficiency are arbitrary - it's really dependant on the metric you are comparing it to. For instance (I'm in fisheries), if you want to efficiently catch fish, use a big bottom trawler, if you want to efficiently manage a fisheries for longterm sustainability - a bottom trawler may not be the best choice (although still sometimes is). My point in the measure of 'efficiency' that people who drive automobiles I would say are most interested in in miles travelled. Sure you could argue that that is not the measure of efficiency you are interested in (perhaps it's acceleration which many fuel 'efficient' car suck at) but lets stay in reality here.

[/quote]
I think this is a perception problem manifested by the arbitrary units you are using. For Joe and Jane Average, the two situations you mention above are mutually exclusive, and don't take this the wrong way - it's a strawman argument.
[/quote]

I guess, but any unit your using is 'arbitrary' I suppose. My point is that the volume/distance metric is much more transparent and more easily used. This assertion is supported by the article in Science that I posted and most governement agencies that are interested in educating the public in these sorts of things. Sure, the EPA stays primarily with MPG (or is it rods to the hogs head?) but they give an option to got to L/100 -- why? because it's better.

[/quote]
You either have a 20mpg car or a 45mpg car you want to improve. Your decision will be how much you want to improve this by. If I had a 20mpg car and I can buy another car that gives 10mpg improvement, I'm not really going to be concerned whether another car's 45mpg actual improvement will be less if I apply the same 10mpg increase.
[/quote]

Yeah... I'm not really getting this, I don't think you are understanding the non-linear problem. It's an important point to consider when developing energy policy. Think about it.

[/quote]
Surely any FE improvement is all good though?
[/quote]
I guess, but I don't simply think of things in black and white good or bad. Details are important - and using the correct tools to make the best choices are needed if we are going to make real cost effective changes to our energy consumption.

 

mpg or l/km
Over here in uk I prefer mpg

However neither take into account speed

What we really need is gallons per hour or litres per minute

 

*cough*
Thanks for catching the mistake. It should have been: fuel consumption proportional to distance.

 

I totally agree, and I think one of the things that consumers (and media) need to keep concentrating more on is not just fuel efficiency, but rather fuel usage - manufacturers can make the Hummer as SFC efficient as it can be, but its huge power output (and hence fuel consumption) is still only justifiable for ferrying troops and equipment in the battlefield: doing weekly shopping or daily ferrying of a kid back and forth to school are not good reasons for having one!

It's interesting that in a news article in Australia, there is a surgeon who insists on driving a Hummer (with a bullbar at the front) because he thinks it's the safest vehicle on the road for drivers/passengers, based on his medical experience... he's obviously not considering any ER patients who have been hit by monster vehicles...

Hummer stands between doctor and drivers - Latest News - inMyCommunity - Perth, Western Australia