What's the Deal...?

Give me metric or give me a headache.

Lots of ways to compute in your head the above conversion if you tolerate some inaccuracy:
I memorize 4.7 L/100km = 50 MPG(us), and calculate from there

Example: 45 MPG is ~10% higher consumption than 50, so the metric answer is 4.7+0.47

Or you might like to start from 6L/100km = 40 MPG(us)

 

Divide 235.2 by either value, the result is the conversion. Not something you do on-the-fly-in-your-head, but hey.

(Worked that out once, don't ask me how it works right now, I'd have to fire up both brain cells.)

 

Let me check if both of mine are working today!
50 (MPG) = 4.7 (L/100km). Multiplying those together gives 235.2, as does multiplying *any* pair of equivalent numbers.
So dividing 235.2 by any number in MPG or L/100km will always provide the correct conversion.

 

Neat! And when you multiply any pair of numbers in the two right hand columns, the answer is always ~235.

 

Just confirming my explanation of Mendel's method.

 

The advantage of a volume/distance is that figuring out average consumption is easier. With a MPG or similar you have to convert to a gal/mile first for a correct answer.
A gal/mile might also have a stronger impact on the individual in terms of consumption awareness. They buy it by the volume, and it lets them see how much they use for a trip with less head math.

 

One of the other "advantage" of a volume/distance system for those eccentric numberphilic engineers like me: an distance/volume system like MPG can never handle extreme correctly. Say your Prius is running in EV mode and is using no gas at all, a distance/volume calculation will give you a infinity (instead of 0 in a volume/distance system). In the Prius, the MPG is shown as 99.9, which is technically not correct. I'm not sure how the trip computer actually handles this condition, but if it is calculating the avg MPG using 99.9 instead of infinity, the result will be off, by not that much though.

FYI: L/100km is used in China.

 

In principle, it really doesn't make a dang bit of difference whether we use distance/volume or volume/distance expresssions, as long as we're able to do the arithmetic. Some people aren't, apparently.

However, the X liters per 100 km custom seems much more cumbersome than mpg, because it requires using a decimal for reasonable resolution, and because it introduces the purely arbitrary number 100. Where did 100 come from? It might as well be liters per 1000 km (which would reduce the need for the decimal) or liters per km (which requires the decimal but eliminates the arbitrary multiplier).

 

I work in imperial mpgs, but am starting to remember the litres per 100 km now.

All of our cars here in Ireland are in Kms now, (new registrations), but can be converted to mpgs too

As I but my cars in the UK it's all on the old imperial system. It's like metres and feet, I just switch between the two

I actually don't mind the litres/100 km if I'm honest.

 

Here's the philosophy: Miles per gallon is great if you're constrained to driving only as far as you have the gasoline for.
In most modern circumstances I'm going to drive as much as I need to and want to know how much it will cost me in fuel.
It also makes consumption more logical to compare. People act as if gaining 10 mpg is a big deal. If you're starting at 15 mpg it is (a difference of 6.3 l/100 km between 15 and 25) but starting at 55 mpg (a difference of 0.7 l/100 km between 55 and 65) it really isn't.

 

I'll tell you one immediate benefit of using amount/distance instead of inverse: computing averages is straightforward.

And along css28's line of reasoning, the opposite effect holds true also, particularly in the arithmetic illiterate US: it makes it harder for gas guzzler owners to brush aside improvements of "only" 1 or 2 MPG. I cannot count how many times I have read people say that their cars get about the same mileage no matter how they drive -- 14 MPG at worse, 16 MPG at best

 

I guess it's better than furlongs per hogshead. The difference between UK and US units just never stopped causing confusion.

 

The reason that gal/100mi is better than mpg is that your fuel spending moves linearly with the natural increment of the measurement.

Example:
Buying a vehicle that is 5mpg better than your 15mpg truck saves you $875/yr (based on $3.50/gal, 15000 miles/yr). If you buy a sedan that is 5mpg better than your 35mpg sedan, you only save $187.50/yr. Both scenarios improved by 5mpg, but one saved a lot more money than the other.

Using 0.5gal/100mi as your increment, going from a 6.5gal/100mi truck (15mpg) to a 6.0gal/100mi vehicle saves you $262.50. Going from a 3.0gal/100mi sedan to a 2.5gal/100mi sedan saves you $262.50. Both scenarios improved by 0.5gal/100mi and both saved the same money.

I think if we used gal/100mi, people would think twice about choosing a truck as their daily driver versus a midsize sedan. You are pretty much paying double to fuel it versus a midsize sedan.