Miles per Gallon

Yes, I've been meaning to add that link at the bottom of the page with a short piece of text for context - I'll do that soon.

Thanks.

Edit: Done that now. Not sure why this comment got a downvote, though.

Extra Edit: Hmm, now been re-upvoted. Bizarre, but thanks for the votes! I know that they're not actually worth anything, but they do indicate that what I've done is appreciated, and considered to be of value.

Thanks for that! Not being a native English speaker I couldn't really grasp what the author was trying to describe when he started talking about a "trough with a cross-sectional area of 0.1 mm2" and the following paragraph. This image made everything clear!

The USA is BIG. Foreigners who come here often have little idea what an unfathomably huge place the USA is. And unlike say, Russia or Canada which are even bigger geographically, virtually all of that territory is navigable by car throughout much of the year.

In the USA, therefore, it was historically pretty likely that grandma and grandpa lived far away, say, back at the farm while mom and dad moved into the big city or the suburbs to get jobs and raise a family. So in America there was a much bigger market for grand tourers -- cars designed to go long distances and be fun to drive. Early consumer automobiles were promoted as such -- "See the USA in your Chevrolet" and the like. Now you know why American cars are reputedly so big and ugly: in Europe GTs are rich boy toys whereas in America they had to appeal to middle-class sensibilities which meant things like big engines and rocket fins on the rear and such.

So in this environment, it made much more sense to market a fuel-economical car not with statements like "you can use less fuel and save money over your trips to the market and to work" -- oil was cheap at least until the 1970s -- but with statements like "you can go this much further on a single tank of gas". Hence the metric of miles per gallon, rather than litres per 100km or gallons per 100 miles -- which is still in use today.

The thing about mpg that is problematic is that it obfuscates the savings in going from a 10-20 mpg vehicle to a 20-30 mpg vehicle (and exaggerates the benefits of a 50 mpg vehicle).

15 mpg -> 25 mpg is a much bigger step than 25 mpg -> 35 mpg (a savings of 2.6 gallons per 100 miles vs a savings of 1.15 gallons per 100 miles). The jump from 35 mpg to 50 mpg only saves 0.85 gallons per 100 miles.

For everything else it just introduces some trivial difference in the arithmetic.

> So you think like this: "distance from point A to point B is 250 miles, my car gets 25 miles to the gallon, so I'll use 10 gallons, 1 gallon price is $3.00 so it'll cost me $30.00". Easy as pie.

That's why. It's exactly the same thing you're talking about.

MPG is useful when you are thinking in terms of vehicle range. This isn't most people's default, but it does have its usefulness.

As for why US doesn't use l/100km. the simplest answer is all our vehicle odometers are calibrated in miles, and we buy gasoline in gallons, so l/100km is a number with no grounding. To get most people in the US looking at it it would have to be gal/100miles or similar.

I'm Canadian and buy litres of gas, but I still check the mileage in mpg. It's just because I have an instinctive idea of how many mpg I should get, whereas I have no instinct for l/100km. I know 35mpg is decent and 50mpg is exceptional, and 10mpg is unacceptable. What does 10L per 100km mean? I have no idea.

> I will never understand why Americans keep using miles per gallon instead of liters per 100 km.

Because:

(1) neither liters nor km or meaningful measures to Americans in general; were Americans going to use a measure of fuel:distance, it might be gallons per 100 miles, but not liters per 100 km, and

(2) Even then, I think there's a pretty strong preference for more-is-better measures rather than less-is-better.

> Miles per gallon is only good if you want to advertise your car

That's pretty much the only thing MPG ratings are used for, manufacturers selling cars. So, its not really surprising that that is what is used.

> But you never do that in real life, you never fill your car with say, two gallons and say "now I'll travel 80 miles".

No one I know ever do the reverse, either; you don't decide "I'm going to go eighty miles, and this car consumes 2 gallons per 100 miles, so I need to make sure the taken has at least 2×80/100 = 1.6 gallons".

> Instead you always know the distance between two points and you want to know how much will that travel cost you.

For people who care about that, there is a reason that auto magazines doing reviews often publish things like total per-mile fuel + maintenance costs for reviewed vehicles. If you just care about fuel costs, it takes about as much work (same number of multiplications and divisions, just changes which figures are being multiplied and which divided) to calculate that from MPG and gas price as it does from gal/mi (or gal/100mi) and gas price, so there's no real advantage to using one over the other for that purpose.

(And, IME in practice, fuel costs are usually a concern with trips where the consumption consideration isn't "how many gallons will I get based on the cars stated ratings" but "how many tanks will it take based on my driving experience", in which case the people using it often are using something like the miles per unit of fuel ratings, but the rating is determined by experience with the vehicle, and the unit is whole fuel tanks.)

If people start thinking of individual car trips as spending money (rather than the generalized cost of having a car), I wouldn't be surprised if people drastically reduced their travel and, correspondingly, consumption (by not leaving their houses). People seem to highly resistant to impulse purchases for dollar amounts smaller than they are willing to spend on road travel without thinking. This would be a Very Bad Thing for the American economy, so I'm not surprised that we are encouraged to reason about gas milage in language that makes this more difficult.

Only on HN would a stupid "herp derp you guys are using the wrong metrics" post be voted this highly. I get it, snotty Eurpeans get snotty upvotes from each other with their snotty, "We dont like your imperial units" whining on the internet. Umm, congrats? The rest of your argument is bunk as its trivial to find out what your gas capacity is and know your max range is. Most people don't worry about how much it will cost to drive to grandma's cost.

Also, gas is cheap and Americans are wealthy. We don't buy one or two gallons and walk away hoping to get through the weekend. We typically fill our tanks and off we go. Long drives are the norm here as you may have friends/relatives in the suburbs or out of state.

On top of it, our geography is HUGE and unlike "big" countries like Russia or China or Canada its almost all drivable/livable/usable. Knowing your range is more important to most people than knowing how much it costs to get to grandma's house. Its in your countries where gas is super-expensive that you have to worry about those things. Not here, at least for most people.

MPG is a perfectly rational way to talk about car efficiency, even in everyday use. Considering the typical IT automation in cars nowadays, this argument is academic as you can trivially bring up up all sorts of info on the console.

Sorry, but the usual "the Euro way is the only and the best way" whining is wrong again. Turns out there are lots of good ways to do things and "good" is relative to the needs and expectations of the user.

I'm British and prefer mpg. Why? Because for starters kilometres are alien to me, but also because it's just familiar and relatable. But with that said, the numbers manufacturers give are never even vaguely useful IMHO (my car is technically 40mpg but I get low 20s) so it's more for comparison of efficiency between journeys than anything else.

It's just as simple doing the same conversion for mpg. If I'm going 250 miles and get 25mph I instantly know that's 10 gallons multiplied by whatever the cost of fuel is

For a while, I avoided using gallons at all. I'd drive a hundred miles, then fill up. Then dollars to fill up = cents per mile.

Now I just put 5 bucks in when I can afford it.

Sorry, this is a non-argument because you are trying to solve a non-existing problem.

The calculation used to determine range or cost when you have miles-per-gallon or km-per-liter is trivial and just as familiar as the liters-per-distance calculation when viewed in context.

My point is that if you actually LIVE in an environment where miles-per-gallon is the norm you develop familiarity with the numbers. You develop a sense of proportion. The calculations are not difficult because they are normal to you. To anyone transitioning back and forth between the two systems, well, there will have to be an adaptation period during which things will be uncomfortable until they are not.

Besides, people very rarely have to make these calculations today. My car tells me how many miles I have left in the tank based on the average historical fuel consumption since I've owned it. No math required. When I fill up, it updates that number.

Still, you very quickly learn that you get X number of miles with a full tank of gas. You just know that. You also learn you need to refill your tank N number of times in the course of a month on average due to your normal driving patterns. None of this requires much thought. And, if you are going on a long trip you generally know how far you can go on a single tank of gas and hopefully you've verified that there are gas stations along the route to top-off as you go (which in the US is generally the case).

As far as calculating the cost of a trip, again, if you live in miles-per-gallon land it is easy. Nobody goes into a crying frenzy screaming "Why, oh why don't we use liters per 100 km. This is so hard!". No, instead people have an estimate of their average miles-per-gallon or an accurate historical average from their car's information display system. They get the distance to their destination and divide. And, no, there's no need to divide 483.5 miles by 21.3 miles-per-gallon. That's insane. Nobody does that. Your goal is to get a sense of proportion of how much fuel you'll need. So, you divide 500 by 20 and you'll get a decent number. It won't be twice that many gallons nor half. There are plenty of gas stations along the route. No worries. And, if you need an accurate number for reimbursement purposes, well, keep the receipts.

So, again, I think you are trying to solve a non-existing problem.

Now, on the question of metric vs. imperial. Well, I detest the imperial system of units. I hate it. I use metric for engineering purposes. The world is metric. It is ridiculous that we haven't made the switch.

That said, I fully understand why. Unless you've traveled through the US it is hard to get a sense of proportion. This place is huge. The infrastructure is of unimaginable proportions. Going metric could very well take 100 years due to the sheer amount of work and cost involved. So, yeah, I get it.

There are other seemingly minor examples of this. We sell internationally so I am very aware of cultural and systemic differences. For example, most of the world would write one thousand dollars like this: $1.000,00. In the US it is written like this: $1,000.00. In the US 1/2/2015 means January 2nd, 2015. In the rest of the world it means February 1st, 2015. In the US we use AM and PM. In a good deal of the world it is a 24 hour clock (which makes far more sense).

There are a lot more examples of this. If we can't fix something as simple as what the comma and period mean when speaking about money or the order of the day and month on a date we are doomed to fail in shifting the US to the metric system.

>I'll never understand why different [organizational unit] does things differently than me. My way is superior because I've formed a mental model of it.

Hi, I actually work in fuel system analysis.

The one issue with the blog post that nobody seems to realize or point out. Is that fuel consumption is measured, and calculated internally as kilograms per kilometer. The change to volumetric units is done for the benefit of the population.

This comes into play because namely, thermal expansion. Normally even in return less engines fuel is used to cool injector heads, as well as some body block parts (depending on engine design), as hotter fuel more readily reacts, and is already being routed to the warmed parts of the engine.

That seems like a coincidence, that the relative velocity between the car and the fuel shooting through its injector should be approximately the same as the relative velocity between the car and the road.

One could conceivably build a car with half-size injector nozzles that squirt in the same volume of fuel per unit time.

> Note also that there are approximately 3.785 liters in a gallon, not 4.5461 as stated in the original article.

"gallon" is a unit of volume in several systems of measures, most notably the US Customary system and the Imperial system (these systems are often incorrectly conflated because many of the units are the same, and where the units are different many of the names are still the same.) The size of the gallon is not the same in those systems:

  3.785 l/gal(US)
  4.5461 l/gal(Imperial)

    ... there are approximately 3.785 liters
    in a gallon, not 4.5461 as stated in the
    original article.

Not if you're in Europe. Which I am.

(0.1 mm^2 is half the cross-section of AWG 24, getting close to hair thin.)

I don't think you're correct here. For round numbers, let's take a 100 kW engine (134 hp), and suppose we feed 100 V. At full power, we'd be running 1000 Amps down that wire. Even at 1 kV it would take 100 A.

Better get a superconductor. Or a much thicker power cable.

How about a pressure differential?

https://en.wikipedia.org/wiki/Atmospheric_railway

Cross-section of pipe = available power

Agreed, the south shore chicago trains have feelers to harvest electricity as they go. Maybe a Tesla with a whisker on top could go indefinitely on long stretches of highway.

If one needs 9000L of air to burn 1L of fuel, that would be 9000(0.1mm^2) not (90000.1mm)^2. 900mm^2 is much saner don't you think. I doubt the car would be able to scoop 900000mm^2 as it is 0,9m2. Can you imagine that amount of air fed into engine? Me neither.

Instead of scooping up the fuel, you could think of it as laying out the fuel in a line behind the car. All the fuel starts off in the fuel tank, and slowly drains out of the car. Fuel efficiency is how large the hole is that the fuel drains out of. In reality, it gets turned into a gas and blows away, but the effect is much the same.

Not really, fuel consumption as an input takes carrying or not carrying fuel into account...same as wind resistance of the pickup or the friction of it moving through a trough of liquid or the force of the liquid moving into the void behind the pickup, etc.

The environmental concerns with having fuel evaporating everywhere would be the first practical problem you would want to solve.

(a small working reservoir handily solves the problem of needing more fuel at some moments.)

The Model S has an EPA rating of ~237.5 Wh/km. I also see that the Model S has a volumetric energy density of ~700Wh/l.

Cancelling, I get ~0.34 mm^2.

Another question would be the mass per unit length. At 250 Wh/kg, that works out to ~0.95g/m. For comparison, the 30mpg car works out to ~0.057g/m.

I suppose it depends on your definition of "car", but they've been doing this for many years in the World Solar Challenge. GM's Sunraycer drove across Australia in 1987 on solar power alone: http://en.wikipedia.org/wiki/Sunraycer.

I think 2*4 will do the job just fine but you may want to check with the folks at http://www.onlysolars.com/ for this.

It becomes important to decide how much it weighs and how fast you want to go (and if it has to work at night in Alaska...).