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Comment by noneeeed | Hacker News Reader

I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car?

I know that such measures are always imprecise due to different driving styles/conditions etc. So what is a better way to measure the efficiency of a car?

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> I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car?

Because it has a sublinear relationship to absolute efficiency: an 11mpg car is significantly more efficient than a 10mpg car, while a 41mpg car much less so in relation to a 40mpg car: both provide an extra mile per gallon, which translates into a 10% and 2.5% increase in efficiency respectively.

> So what is a better way to measure the efficiency of a car?

gpm (aka L/100km which is what's used in most metric countries).

It's less convenient when you know how much fuel you have and want to know what distance you can expect, but it's way more convenient to compare efficiencies, as well as to calculate how much fuel you'll need for a given trip: the examples above are 21.4L/100, 23.5L/100, 5.7 L/100km and 5.9 L/100km. From this, it's obvious that the absolute gain in efficiency is much greater going from 10 to 11 (2L/100km economy) than from 40 to 41 (0.2L/100km)

Ah, I see, that makes a lot of sense, thanks for the explanation.

Well, even if the better unit is liters per 100km, you’d still face the same problems because the UK/US would be talking about gallons per x miles where they would still face the issue the poster described of differing units of gallons.

The point I was rather explicitly (if you go read the original comment) making is that mpgs are bad for comparing fuel economies regardless of differences in what is meant by "gallon", because it's intrinsically not a good unit for comparing fuel efficiencies.

I tried to explain this, but it turns out to be kind of hard, so instead I found a reference for you: https://www.bunniestudios.com/blog/?p=257

Basically, it makes it easier to compare fuel economy differences at different points in the scale because they become subtraction problems instead of division problems. This is because usually the amount you are going to drive is fixed, and MPG puts the miles in the numerator, while "gpm" puts the fixed miles in the denominator, I think.

Only if you don't drive for fun. If you do, it's a lot more useful to know how far you can get on a tank.

The original comment was about comparing fuel efficiencies.

But to your point: mpg makes it easier to know how far you can travel on a given volume of fuel, L/100 makes it easier to compare fuel efficiencies, and to know how much fuel you would need for a given distance to travel.

To me the latter seems generally much more useful than the former. The former provides an easier answer to "how far can I go burning this tank", which feels like a much rarer use-case than "how often will I need to refuel to my destination" (and "can I reach my destination on a single tank") for the general population.

The best way I've heard it phrased is that europeans 2ant to know much much fuel they need to get to their destination, while americans want to know how far they can go on a tank. Its obviously oversimplified/not relevant/other pedantic nitpicking but it captures the different relationship americans in general have with driving and their cars

Maybe this is because most people with cars in America (especially in the 1950's) were not worried much about the price of gas. Europe had much higher gas prices and was much poorer, so how much it would cost (how much gas would be used to get somewhere) was more important for planning than knowing when they would need to fill up again.

With volume per distance, you just need to divide the fuel tank capacity by the volume and then multiply it by the distance to get that. With distance per volume, you have to multiply the distance by the total volume of the fuel tank. The first calculation isn't that much harder than the second.

Realistically though, once you've driven the vehicle for a while, you pretty much know the total distance it will get in a tank regardless of the way fuel usage is measured.

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