Mentality behind terminology: MPG vs L/100km

MPG is very misleading...

It places focus on gallons SAVED rather than what is actually USED.

...hence the preference for unit/distance measure instead. Heck, even the EPA provides efficiency estimate information that way now. Look up a 2009 Prius at Fuel Economy and click on the "Gallons/100 Miles" option.

How much fuel it takes to travel a specific distance makes a whole lot more sense.
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I don't see where the mislead is on mpg vs. km/L - these two measurements give the same exact metric in terms of fuel efficiency. There are many ways you can provide an indication of fuel efficiency but provided you are consistent in using the same measurement parameter while comparing vehicles, they are all useful.

If you are talking about SPECIFIC fuel consumption, which is what we all need to look at to determine absolute measures of fuel to propulsion energy efficiency, this is something else altogether and needs to be calculated with other parameters such as mass, speed and temperature.

I think the EPA use distance/volume measurements simply so the conversion to European Standards - comparisons between European and US cars - is easily done. Globalization and all that...!

 

Those would be the same metric. They were talking about mpg vs. l/100km. The first puts the emphasis on how many miles you can squeeze out of one gallon; the second on how many litres of gas it takes for you to go 100 km.

 

We're the country that calls our national Baseball competition the World Series, as well as the country that gave birth to the Miss Universe pageant, so criticize all you want - you'll probably be correct.

 

My mistake. I meant mpg vs L/100 km. These two still provide you with the same metric, which is fuel efficiency. I contend that they are merely two different ways of looking at the same indication. After all, in coming up with either mpg or L/100km figure for my Prius, I'm still talking about the performance of the same vehicle - its fuel efficiency is the same!

 

This should help clear things up.

The author of the MPG illusion on why we misunderstand the meaning of miles per gallon (and why we shouldn’t completely hate the hybrid Cadillac Escalade) Nudge blog

Converting things to volume/distance is much easier to understand and much more transparent. MPG can be very misleading. For instance, the gas saved by a prius over an new insight are pretty much meaningless in comparison to hybrid SUV's. Interesting to think about.

 

I drove several times in Germany- Austria etc finding the Euro Benzine (Gasoline) efficiency model rather confusing. One never new if personal driving habits as well as personal driving efficiency resulted in mileage fitting the posted benzine efficiency. Now that I see Euro cars such as BMV / Volvo in the US, I realize they are not efficient autos.

 

I think using the term MPG is proactive and encourages conservation...I like it. It also provides an incentive for high-MPG vehicles on the lot for consumers who actually wish to get better mpg vs. buying a car for what it looks like or how "cool" it is.

Different vehicles may have different size tanks and one guy getting 60 mpg and driving 500 miles on a tank is doing better than a guy with 20 mpg and driving 500 miles (this was the case with my former 22 gallon 4runner and my 8-11 gallon Prius...I might be stopping by the gas station the same amount of times but I'm also saving $10-15 each time I do it).

Unfortunately, most US drivers care nothing about mpg or even how much they pay at the pump. They either think they have no control on how much they pay at the pump or they don't care when in reality fighting pump prices with increased mpg is possible.

 

I think it's only meaningless if one's scale of comparison is different... kinda like me saving $100 over a year is a big deal to me, while it may be meaningless to someone like the Treasury which wants to save billions of dollars a year!

The other idea is that to truly make a meaningful efficiency comparison between different automobiles, specific fuel consumption related to load and conditions is the only metric that will ever make sense. Even volume/distance is inaccurate and misleading when one is determining absolute efficiencies. Car weights are different for one, environmental conditions in different countries are two, and typical loads are three, different depending on purpose for buying (e.g. SUVs vs two-seaters).

 

Fuel Efficiency measured in liters/100 km or gallons/100 miles is far more rational than the old traditional MPG measure. It truly measures fuel consumption efficiency, relative to traveling a given distance, as opposed to miles traveled efficiency, relative to consuming a given volume of fuel. What you start with is the fuel, not the miles!

This is further reflected in the way the Prius measures MPG, where when the ICE is off, the MPG figure defaults to 99.9, which should really be infinity, since any finite number, in this case a distance, divided by zero is an infinite number. But I doubt many people, other than mathematicians or engineers would understand the "oo" symbol. When L/100km, or USG/100mls is the measure, the ICE-off figure is zero, an easier number to handle.

This of course omits whatever electric energy is being consumed for propulsion, since it too originates from the gasoline being burned. Measuring the consumption of that amount in terms of MPG or L/100km gets complicated however. But plug-in vehicles really need to take it into account, since they will really have 2 energy sources to pull from, at varying levels of efficiency.

 

This doesn't make sense to me. How else could you interpret for example 4.8 L / 100 Km.

The problem with MPG is that you save a lot more gas moving from a 20MPG car to a 30MPG car (a 10MPG difference) vs a 45 MPG to a 55 MPG car (still a 10 MPG difference).

Changing it to L/100 Km it is clear.
1) 11.9 L/100 vs 7.9 L/100 -- a savings of 4 L per 100 Km
2) 5.3 L/100 vs 4.3 L/100 -- a paltry savings of 1L per 100 Km.

That why there's not much point getting excited about the fuel efficiency difference between the insight and the Prius - that far up the fuel efficiency scale it's not a huge deal. Also, it makes it clear that making large vehicles into hybrids (for those that insist on driving large vehicles) can make a significant contribution to reducing gas usage.

Sure you can do it in your head working our relative percentage differences, but why bother? MPG is a measure of fuel efficiency but is opaque enough that the average person has a difficult time making comparisons. I am of course talking about your average undergraduate student who has piss poor math skillz.

 

Folks, MPG and liters per 100KM are simply mathematical reciprocals of each other, once you adjust for units--miles vs. 100KM units, gallons vs. liters.
Miles per gallon is distance/fuel volume; liters/100KM is fuel volume/distance.
There's simply no reason one is "naturally" better than another.
Ken

 

"We realized that the mpg scale was not intuitive."

I find that most Americans don't find numbers or math very intuitive, regardless of how it is presented.

And as an engineer and numbers guy who can work with either measure, I'm also the wrong person to ask about how to fix any perception problems to help the average public understand it.

 

Read above post.

By your reasoning the imperial system is equivalent in utility as the metric system which it is blatantly not.

 

Again, I don't think mpg is superior to L/100km, or vice versa... these really are fundamentally just reciprocals, and the only important thing to consider is consistency, i.e. using the same parameters in comparisons.

Besides, neither measures true fuel efficiency - to do that, you would need to measure fuel burned versus power output, not distance travelled. Distance travelled for a given amount of fuel burned (or v.v.) depends on load, road conditions, atmospheric conditions, vehicle configuration, tire condition, engine condition and so on... in other words, the L/100km (or mpg) figure you quote to me one day doesn't tell me anything about the inherent fuel efficiency of your vehicle - it only tells me how far you got with what amount fuel for your particular car for that particular day over that particular course you went through with your particular driving technique. That will not necessarily be reproducible or typical or comparative of all the other Prius' out there because I don't know how you drove your car, over what roads, how rough the surfaces are, at what temperatures and pressures, with how much load, at what vehicle condition and how deteriorated your engine is, among other things.

At best, mpg and L/100km are rough comparative indicators of the same thing, and this is why EPA specifies strict standardized conditions for determining mpg or L/100km figures, and why the EPA figures do not translate well into real world usage.

To me that is just preference, kind of like whether you express absolute zero temperature as 0 K or -273.15 C or -459.67 F... it depends on what you're doing with the parameter.

 

However, there is a perceived difference... which can be taken advantage of... and greenwashers do.

Remember, there are lots of people that don't bother to actually do the math. They just see bigger & smaller numbers and make assumptions.
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For the math adept, such as those of us whose careers revolve around it, this is true. But most of the population does not fit into this category.

 

Exactly. I also don't care what number base we use, or whether angles are in degrees, radians, or grads. Just tell me what you are using.

Not everyone is comfortable with math and units. Much of the U.S. population needs remedial mathematics. As a country and a culture, we need to do something about it, but that's another topic.

As for me, the two are reciprocals and I don't care which you use.

Tom