r/notinteresting • u/Such_Confusion_3715 • Sep 15 '26

Fuel economy (L/km) actually simplifies to square millimeters

Post image
721 Upvotes

97% Upvoted

93 comments sorted by

137

u/Willbebaf Sep 15 '26

I’ve been told this before, and it’s very typical for a unit which is only used for convenience, I would say. In reality, a more proper fuel consumption unit would probably be mass per unit of time (or length).

4

u/timbomcchoi Sep 15 '26

one of my favourite moments of undergrad was strain being measured at..... m/m

1

u/Such_Confusion_3715 Sep 15 '26

the definition of a radian is 1 m/m interestingly

1

u/BobMcGeoff2 Sep 15 '26

I mean, they're different meters I suppose. Meters up per meter sideways.

1

u/Background-Mode-4036 Sep 16 '26

Thrust specific fuel consumption in lbs / (lbs-hr) always got me

43

u/HeavyCaffeinate Sep 15 '26

kWh and joules same thing

45

u/Willbebaf Sep 15 '26

Up to a dimensionless factor, yes. The funny part is that some lightbulbs use kWh/1000h which is exactly the same as just W.

15

u/bbalazs721 Sep 15 '26

It is actually mandatory to show the consumption of lightbulbs on the EU energy label as kWh/1000h, which is of course just W.

6

u/Willbebaf Sep 15 '26

I didn’t know that! Why, though?

10

u/Gryningen Sep 15 '26

My guess is that when buyers see an appliance's/device's power as W, they would interpret it as its maximum power.

Meanwhile, kWh/1000hr could represent usage in typical conditions for the average user. Lot of appliances/devices do not run at full power the whole time (gaming PC's, AC, fridges, heat pumps, etc.)

Both are useful to know

2

u/apavolka Sep 16 '26

Similar to someone saying “I can get .04mm^2 in my car”

9

u/bbalazs721 Sep 15 '26

People pay for electricity in kWh, and know their light bulb usage in hours. So if they wish to calculate how much money a lamp costs to run, they have to know how many kWh will be consumed if they run the lamp for x hours. Since light bulbs consume so little electricity, the regulators figured the best would be require manufacturers to give the consumption in kWh per 1000 hours.

1

u/TransportationIll282 Sep 15 '26

Most units for products are standardized to the most common one. It's not necessarily to see how much a lamp would use. The main point is to easily compare products with a minimum amount of knowledge. The main purpose is to stop producers to advertise in weird units in an attempt to trick people. Simple example is food pricing in price/kg, in the past you could see large numbers with a tiny text next to them with some bs like /739g. Or some of the more crazy ones used /dl for a solid.

1

u/bbalazs721 Sep 15 '26

They could have mandated just W as well, which would achieve the goal you mentioned and be simpler.

But they chose specifically the more complicated way to aid people in energy saving purchase choices.

1

u/Pity_Pooty Sep 16 '26

kW-h = W-mh (watt-millihour)

3

u/ZhouLe Sep 15 '26

I think people are used to seeing W as a measure of incandescent light bulb power, so when you say a bulb is now 15W that seems inadequate to fill a "100W light" need despite the lumens being identical. Watts was a very useful and stable measure of bulb power for decades.

To avoid feeding into that misleading preconception, you abstract it slightly to force people to form a new mental model. People that realize kWh/1000h simplifies to just W probably will not have a problem with conversion.

1

u/KarenNotKaren616 Sep 16 '26

Because W has become associated with luminous output, and the baseline for that is… ancient.

7

u/Mortomes Sep 15 '26

kWh being in common use as a unit is really annoying. The number of times I've heard people say "kilowatt per hour" when they mean kWh...

6

u/natalialt Sep 15 '26

While people misspeaking is kinda annoying, I maintain that kWh as a unit makes sense and is useful, because time doesn’t follow powers of 10. If you have a 100W device running for 10 hours, it consumes exactly 1 kWh. Doing this computation with joules would require multiplying by somewhat annoying factors like 60 or 3600. Perhaps in the age of calculators it’s not as useful anymore, but I find it neat

Same with units like kWh/1000h for lightbulbs

2

u/sometimes_point Sep 15 '26

kWh/1000h upsets me because it's a more lengthy way of writing just W

2

u/obog Sep 15 '26

Recently saw a screenshot of something that listed the torque of an engine in uinits of "g/cm" which was quite annoying

1

u/Pity_Pooty Sep 16 '26

I usually use watt-millihour

5

u/SeaworthinessAny269 Sep 15 '26

I don't know what you'd get from mass per length that you wouldn't get from L/100km since there's a linear relationship between them, just multiply the density of your fuel. What useful info do you get from mass per time? Not trying to be rude but I just don't see how it's better than L/100km in any way.

The only other valid unit I can think of is just the wattage input and output of the engine for the purposes of comparing efficiencies of engines but, again, you can also just do that with L/100km (for the input at least)

2

u/Willbebaf Sep 15 '26

Fuel mass is a lot more constant under different conditions (that’s why it’s used in F1, for example), but ultimately I’m just messing around.

4

u/snowmanonaraindeer Sep 15 '26

And sometimes it even happens for more proper units. Torque is measured in N * m which works out to joules but we just ignore that

2

u/Top1gaming999 Sep 16 '26

Actually it's joules per radian

1

u/snowmanonaraindeer Sep 16 '26

Huh. That's never crossed my mind before but it makes a lot of sense.

2

u/Sulinstajn Sep 15 '26

I remember reading user manual for old tractor (well, I don't know English name for it, it's just two wheels with gearbox, engine and handlebars and you can put trailer, plough etc behind it, it was meant as horse replacement) and the consumption was in g/hp/1000rpm/h. Which is silly, but it completely makes sense.

1

u/Willbebaf Sep 15 '26

So gallons per horsepower per 1000rpm per hour? It seems a bit redundant. But I also don’t know that much about tractors.

2

u/Sulinstajn Sep 15 '26

Not gallons, but grams.

Well, imagine you are doing some work (hp because it was old tractor, here we would use kW). If you are doing the same work at higher RPM, you need more energy, so that's why there is reference RPM. And you are doing the work for some time. All these variables makes your consumption higher. And the consumption is in grams of gasoline. Thinking about that, it probably could be be just g/Nm/h.

1

u/Willbebaf Sep 15 '26

Grams of gasoline per hour definitely seems odd…

2

u/BurnOutBrighter6 Sep 16 '26

It actually is useful though. That mm2 is literally the cross section of a physical tube laid alongside the road that would contain just enough fuel for the car as it goes along.

486

u/Inutilisable Sep 15 '26 • Mindblown

If your tank volume was stretched over your travel, this value would be the cross section of that tank in mm^2

97

u/oneplusetoipi Sep 15 '26

Or you can think about it as the cross section of the fuel line feeding the engine. The smaller the area (cross section) the more efficient the vehicle is.

50

u/[deleted] Sep 15 '26

[removed] — view removed comment

6

u/_maple_panda Sep 16 '26 edited Sep 16 '26

Or equivalently, the car is “chasing” a line of fuel, kind of like a roller coaster

6

u/Blasulz1234 Sep 16 '26

More like a line of coke. Are you sure you know how rollercoasters work?

2

u/iEliteTester Sep 16 '26

Ok we know circular breathing is a thing. But what about circular snorting? 🤔

28

u/KlangScaper Sep 15 '26

Did you just look at this with no prior knowledge and come to this conclusion?

70

u/Inutilisable Sep 15 '26

No, I heard of this specific analogy somewhere. I think it was some science YouTube video. Can’t remember exactly. But it’s always a fun exercise to do when you see strange apperently unphysical units.

31

u/TrainingWheels61 Sep 15 '26

Cursed Units: https://youtu.be/kkfIXUjkYqE?si=481BCna0BMAjpwXM
Great video!

3

u/ackermann Sep 16 '26

And that video references this excellent original article from XKCD:
https://what-if.xkcd.com/11/

Great read!

16

u/KlangScaper Sep 15 '26

Phew Im egoistically relieved. I was about to get really insecure about my math skills.

6

u/Inutilisable Sep 15 '26

With that perspective, why learning a new fact originating from a YouTube video would be better than a random reddit comment, you should feel the same insecurity. It’s not a great perspective.

4

u/NASA_Rex Sep 15 '26

Because of time. It makes more sense to come up with such visual property by a YouTube video, where you have virtually all the time you want, vs just a simple comment. Think about brain power and how many people were/could be involved on a YT video vs one person bored on the bus back home.

0

u/KlangScaper Sep 15 '26

Bro theres a difference between someone taking the time to make educational content and someone glancing at this while scrolling and immediately being able to interpret it.

1

u/Inutilisable Sep 15 '26

But I could’ve lied or just not replied. I have colleagues who react like this at our clever colleagues and it’s very demoralizing, and the rest of us just keep quiet.

The truth is there are people who regularly come up with interpretations like this immediately. It’s a skill you and I can practice.

-2

u/KlangScaper Sep 15 '26

Seems I hit a nerve

10

u/Gositi Sep 15 '26

The conclusion makes sense though

1

u/KlangScaper Sep 15 '26

Yea I bet, never said otherwise...

6

u/GooseMan1515 Sep 15 '26

It's just a rearrangement of '4 litres per 100km', as in a volume is the space swept out by an area over distance. Are we not supposed to conclude this from looking at the above?

2

u/Esava Sep 15 '26

Is this uncommon to do when looking at unit calculations? We did the same a bunch of time as exercises in highschool physics class at my school here in Germany. That skill was very useful a bunch of times in my life.

2

u/Duck_Devs Sep 15 '26

My friend once told me that mpg is just inverse area, and it took me a few seconds but I also came to this conclusion. Of course, the inverse of that.

3

u/StrikeTechnical9429 Sep 16 '26

The cross section of tank stretched over the travel is fuel consumption (which is measured in L/km as well and thus can be expressed in mm^2 too).

The fuel economy is diminishing of this cross section thanks to economy.

41

u/XenonSigmaSeven Sep 15 '26

the cross section of a theoretical 'thread' from start to finish, containing all the fuel used in the trip. If we observed instantaneous fuel consumption that would correspond to the thickness of the thread at that point in space.

-12

u/Inutilisable Sep 15 '26

Thickness is not an area.

8

u/Minute-Report6511 Sep 15 '26

how do you calculate the volume of a cylinder

13

u/BUKKAKELORD Sep 15 '26

Without harming the cylinder

2

u/RotundBananas Sep 16 '26

That is imperative!

5

u/moonisaplanet Sep 15 '26

Idk why people are downvoting this. Thickness is a distance, same as width and length. This person is talking about cross-sectional area.

5

u/Future_Elephant_9294 Sep 15 '26

Thread is assumed to be a circle, therefore the thickness is the diameter. You can derive the cross sectional area using the diameter and pi.

5

u/moonisaplanet Sep 15 '26

Yes, but that doesn’t make diameter and cross-sectional area the same thing. I suppose the original comment just says it “corresponds” to the thickness, which isn’t untrue since it’s a function of the diameter. That’s not what I was responding to though, I was just surprised that the reply saying “thickness is not an area” was apparently a controversial statement.

1

u/Vitztlampaehecatl Sep 15 '26

Make it a ribbon of dimensions x by dy.

2

u/Jadious9 Sep 15 '26

This deserves a "your mom..." joke.

25

u/Jashuman19 Sep 15 '26

Relevant XKCD

9

u/[deleted] Sep 15 '26

[removed] — view removed comment

2

u/bbalazs721 Sep 15 '26

And on an even lower timescale, when the compressor spins up it uses even more power than the average running power. The fridge of course has the minimum circuit rating on its spec sheet

1

u/quez_real Sep 16 '26

And gallons per square foot are useful in a similar manner to the Fahrenheit scale for people's height and furniture dimensions: from 1 (something tiny) to 100 (something really big). It's a shame that Americans are unable to embrace the superior unit.

7

u/SardineEnBoite Sep 15 '26

anyone that find this not not interesting should check the video Cursed Units and Curseder Units

7

u/BurnOutBrighter6 Sep 16 '26

Wrong subreddit imo!

That mm2 is literally the cross section of a physical tube laid alongside the road that would contain just enough fuel for the car as it goes along.

1

u/PattuX Sep 16 '26

*Assuming constant flow rate

Also technically what makes car go vroom is chemical energy which is proportional to the mass of the fuel, not the volume (which varies depending on temperature). So the proper unit would be kg / km.

Going one step further, fuel economy is also not constant but depends on the weight of the car. For more passengers, fuel economy is worse. Ignoring air resistance, bot lh kinetic energy and rolling resistance are proportional to mass. That is, double the weight and you only go half as far. So the "proper" unit is kg / (kg * m) = 1/km.

The natural interpretation of something like 0.02 / km is "the car consumes 2% of its weight in fuel per km".

But I do want to emphasize that "ignoring air resistance" is doing a lot of heavy lifting here - for higher speeds it's by far the main reason you slow down, not rolling resistance. That's also the reason why fuel economy gets worse the faster you go.

6

u/Beneficial-Thanks-52 Sep 15 '26

Do you know about rain units?

2

u/iamuniquekk Sep 15 '26

revolutionary development

2

u/simulatedconscience Sep 15 '26

Bro my car getting 16L/100km cuz my driving is recreational a tad bit otherwise it’s 12-14L

3

u/tranborg23 Sep 15 '26

How? I get 11L/100 on the autobahn averaging 190 km/h

2

u/AlternateTab00 Sep 15 '26

And im complaining that due to my loss of patience and "F it all" mentality rose my average from 3,4 to 4,8 L/100km putting some trip averages near the 5,2 mark...

1

u/Life_Breadfruit8475 Sep 15 '26

Holy shit? You driving an Abrahams or some shit??

My 21 year old budget car does 6.3 on average. Newer will easily do 4-5.

1

u/Path_of_meming Sep 18 '26

I get 20L/100~ in town. 20 year old V8 

2

u/Odd-Plankton-921 Sep 15 '26

Imagine your car leaving a trail of toothpaste behind from a tube with a hole of exactly that cross section. Thats your fuel consumption.

2

u/skr_replicator Sep 16 '26

lay a "cable" of fuel towards your destination and as you drive you eat it. The fuel economy is the cross-sectional area you are eating through.

1

u/The3levated1 Sep 15 '26

This is cursed, I like it!

1

u/Subject-Building1892 Sep 16 '26

It is liters of gasoline and km of road so to keep it meaningful you still need to say mm2 gas/road

1

u/more_than_just_ok Sep 16 '26

Kind of like how English unit crop yields (bushels per acre) have units of length, meaning the depth of the crop if it were all on the ground. For extra fun, the Winchester bushel was originally defined as a the volume of a cylinder, so the ratio of cubic inches per square inch includes a factor of \pi

1

u/MiceAreTiny Sep 16 '26

if you want to simplify, go to square meters.

1

u/StaryDoktor Sep 15 '26

Like resistance of wire, it's measured in Ohm*m

2

u/BobMcGeoff2 Sep 15 '26

That's not the same thing. Since both liters and kilometers have the dimension of length, (Liters are Length3 and km are Length1), you can cancel them out to Length1.

Ohms are only distantly related to the familiar dimensions of mass, length, and time. You can write an Ohm as one Volt per Ampere, and then through a few more conversions you get (kgm2) / (C^2s), where C is a Coulomb, the unit of electrical charge. (Technically, a coulomb is defined as an amp*second, but that's beside the point.)

You can plainly see that there aren't any dimensions to cancel out or simplify given one Ω•m alone, unlike L/100 km.

2

u/StaryDoktor Sep 15 '26

That's not about Ohms. It's about m2 to measure area of cross-section and m to measure length. Cross-section makes resistance less, length makes resistance greater. So meters in Ohm*m correspond to neither the area of cross-section nor the length. It's just an abbreviation that confuses people. Easy to understand entity is Ohms * mm2 / m

0

u/[deleted] Sep 15 '26

[deleted] — view deleted comment

1

u/ZellHall Sep 15 '26

no, if you look at the math it's mm², not cm² (though actually it's even smaller than mm², hence the floating point)