Cars in Canada really do get more miles to the gallon. Well, “did”, technically, since Canada now uses liters/100 km for fuel efficiency. The reason why has nothing to do with climate or driving habits and has everything to do with the amusing flaws of the measurement systems that preceded metric.
I like metric, mostly because its consistent use of base-10 makes mental calculation easier (the mass of the water in a 50m swimming pool that is 10m wide and 2m deep is 1,000 tonnes, and the volume of my body is about 80 liters), but also because the systems that it replaced were a god awful mess – worse than most people realize.
The transition from old measurement systems is often thought as being a change from one system to another, but that is incorrect. It was actually a change from multiple different measurement systems to a single unified measurement system. Gallons, ounces, and even the humble inch were either never standardized, or were standardized much later than people realize.
The gallon and the fluid ounce
The main reason that cars in Canada can go farther on a gallon of gas than cars in the United States can is simple – the imperial gallon that was used in Canada is bigger than the US gallon. The imperial gallon is 160 fluid ounces, and the US gallon is only 128. The US fluid ounces are bigger than imperial fluid ounces (more madness!), but this isn’t enough to compensate for there being 25% more of them in the imperial gallon. The imperial gallon was originally defined as the volume of ten pounds of water. I don’t know how the US gallon diverged. The US gallon is now defined in terms of cubic inches.
Let’s all agree to ignore the US dry gallon which has a volume somewhere between the US gallon and the imperial gallon.
The inch
The inch is defined as being exactly 2.54 cm. But it was not always so. The imperial inch used to be slightly shorter than this, while the US inch was slightly longer. This is the other reason that cars in Canada used to to get more miles to the gallon – the miles (63,360 inches long) were ever so slightly shorter.
It wasn’t until 1959 – relatively recently – that the US and imperial inches were adjusted to meet (roughly) in the middle at 2.54 cm. This made the US inch 2 millionths of an inch shorter than the old inch. This is insignificant for building houses but matters when you’re doing large-scale surveys – a US survey foot is different from an international foot. Meanwhile the imperial inch got 1.7 millionths of an inch longer, but UK surveys were done in meters so it didn’t matter.
Other measurements
The size of a barrel depends on what is being measured. The same is true for ounces, although not to quite such an extreme level.
And don’t get me started on the pound (mass), pound (force), and the slug.
Metric measurement malleability
The meter was originally defined to be one ten-millionth of the distance from the equator to the north pole, then there was a canonical meter bar, then it was defined in terms of the wavelength of light, and now it is defined as the distance travelled by light in 1/299,792,458 of a second – which means that the speed of light is exactly 299,792,458 m/s. Over all this time it didn’t significantly change its length – its definition just got more precise.
The gram (one thousandth of a kilogram) used to be defined as the mass of a cubic centimeter of water, but now it is defined in terms of the International Prototype Kilogram – a platinum iridium alloy cylinder. This is problematic because it seems to vary in mass over time. Oops. Efforts to define it more fundamentally are ongoing.
The liter is simple – it’s 1,000 cubic centimeters. That fact together with knowing that a liter of water has a mass of almost exactly one kilogram is what leads to the mass of my introductory pool’s water being 1,000 tonnes, or 1,000,000 kilograms.
Metric status
It’s often said that the US is one of the only countries that has not moved to the metric system, but life is more complicated than that. There are about a dozen countries that still use the US gallon for fuel, and the UK uses miles (m) for road distances while using meters (m) for road clearances. That’s not at all confusing.
Does it matter?
Having the same unit have different sizes in different countries can easily lead to confusion. Gas is typically cheaper in the US than in Canada, but having a smaller gallon exaggerates this difference. And, those along the border who may see ads from both countries probably got confusing messages about the expected mileage of cars. Getting rid of the imperial gallon was great progress – now we just need to get rid of the US gallon.
See also this rant about the different meanings of “1 cup” as a measurement.
Random ASCII - tech blog of Bruce Dawson 
“The kilogram used to be defined as the mass of a cubic centimeter of water” – I suspect you mean either “the gram”, or “mass of a cubic decimeter of water”. 1 cubic centimeter of water is 1 gram. Love the blog!
Good catch. Thanks. I’ll fix right away.
Here in the UK, car fuel consumption is still advertised in miles per (Imperial!) gallon, but dispensed in litres… So my car reports it does 35 miles to the gallon on the last 50 litre tank of fuel I used…
We’re a real half way house between metric and imperial… I can ask for a 1/4lb of ham at the deli, while its shown as the cost per 100g. At least we got rid of pounds shillings and pence in the seventies!!
http://www.acpo.csiro.au/avogadro.htm or alternative https://en.wikipedia.org/wiki/Watt_balance
Now see, it’s all very simple, just use this diagram for lengths:
This for mass:
https://upload.wikimedia.org/wikipedia/commons/1/1e/English_mass_units_graph.svg
And this for volume:
There’s probably one for area as well, but I couldn’t easily find it.
Since the numbers are different, this allows you to use different parts of your brain and increase your skills in both mathematics and memorization.
Of course, these are just the English versions. I’m from Norway, and our traditional inch was different from the English inch (ours was ~2.615cm – roughly the size of a piece of wet wood that would dry to one English inch).
Thanks for the reminder of the many other inches, etc. I think those diagrams are a bit oversimplified however. 31.5 gallons to the barrel? But what type of barrel? There seem to be many sizes, depending on what you put in it.
You talked about tonne (1000kg), but you could have mentionned long ton (1016.047kg) or short ton (907.1847kg)… [https://en.wikipedia.org/wiki/Ton] and then understand some industrial failures 🙂
We should all switch to https://en.wikipedia.org/wiki/FFF_system
I’m sure when I was in Ireland a few years ago the distance markers were sometimes miles and sometimes kilometers leading to a lot more confusion than had they been all one or the other.
Northern Ireland is part of the UK and uses miles, the Republic of Ireland is a separate country and is entire metric so uses kilometres. They share a land border and when you cross, there are signs on the road warning that speed limits are switching to the other system.
Yep, same thing at the US/Canada border.
As a German, I am fascinated by how you guys get anything done with this mess. Here, it is very simple because you only have one base unit per measurement system and prefixes do the rest. The only thing slightly irritating is the decimeter (e.g. 10cm) as this is not used in everyday life. However, without it, you would have a jump between cm and m that would break the whole beauty of the system.
For conversion, you only have to remember certain conversion rules (e.g. 1l of water = 1kg = 1000m³).
Needless to say, the whole chapter of unit conversion and measurements was a very quick affair in school.