If I'm writing in US English, am I not allowed to use the metric system?

Just saw your edit:

However, I'm using it simply just to convey to the reader "the car was going super fast" in a fancier way.

In that case, I'd say use neither mph nor km/h. When you see a car going fast, or sitting in a car going fast, you are not making a speed measurement/looking at the speedometer in order to determine that you are super fast (you may do that to determine that you are still in the allowed limits, though). Instead you are experiencing the speed. You will notice the unusually strong turbulence. You will hear the strong wind noise. You may hear how the motor is at its limits. You may feel frightened by the speed (in particular, if you are not the driver; actually, that may be a time when you may actually look at the speedometer, in the hope that your impression is just wrong — only to see that in reality it's even worse than you thought, as the pointer is on the upper limit of the speedometer). Or on the opposite, you may particularly enjoy going super-fast, thinking how you wouldn't have the courage to do that when you are driving yourself. If you know the driver, you may wonder if he really can handle that speed.

Or as driver you might note that the gas pedal is pressed down to the floor, how you are using every last of the horse powers the car provides, how you leave other cars behind you (that BMW driver you just overtook certainly must be green from envy!). Or you might be concerned that even though you have a really powerful car and are surely breaking all speed limits, you may still be too late to wherever you are trying to get so quickly.

posted over 6 years ago

CC BY-SA 4.0

celtschk‭

740 reputation 16 53 99 13

I would go in the system that you are used to.

This will ensure that you will give the correct values and that you don't make a conversion mistake. I usually stick to the metric system in conversations unless I am sure about the conversion. For written communications it is different, since you have time for a conversion.

(For example I know my height in feet so ill use ft there)

Side note: The US is based on the metric system also. They just don't know it usually. One Inch is defined as 25,4 mm (which is metric)

posted over 6 years ago

CC BY-SA 3.0

Heiko Hatzfeld‭

5 reputation 0 1 1 0

Whether you're allowed: yes, you're allowed to use metric in US English.

Whether it's a good idea depends very much on details of the setting, and of your intended audience.

If your story is set in the rural United States in, say, the 1950s, none of your characters are likely to think in metric units; even a scientist who regularly uses metric in his work would be accustomed to mentally translating, or doing his work in metric, but using US Customary for daily life. If your setting is a starship with an internationally derived crew, it might well be that only the elderly Americans among them would still think in pounds, miles, and Fahrenheit.

Then again, if your intended audience is a scientifically literate America, at least they'll know that 140 km/hr is well above normal highway speed (for America), they will recognize that "subzero" temperatures might only mean below freezing (as opposed to Arctic conditions), and they won't be confused to hear someone's body mass given in kilograms or height in centimeters.

Bottom line: as long as your usage is consistent with your setting, and your audience won't get lost (and you use the units correctly -- I recall a novel where a "roaring gale" was given as "above 35 km/hr"), you should be fine.

posted over 6 years ago

CC BY-SA 3.0

Zeiss Ikon‭

90 reputation 2 5 16 0

If the majority of your readers are from US, then mph will be more easily interpreted. If part of the goal is to make people feel like they are in Italy, then metric would be better. Context could help readers, an accident on the freeway allowed bob to creep along at 4 kph, while Marry zipped through the school zone at 55 kph and had to pay a speeding ticket. The issue is both with the units, and with the laws. So the speed through a school zone is 25 mph in the USA, but that does not mean that it is 25 mph (40 kph) everywhere in the world.

Autobahn speed record 268.8 mph (432.59 kph) according to one web site: https://auto.howstuffworks.com/5-fastest-speeds-on-autobahn10.htm

Really good tries are slightly over 200 mph.

Wikipedia has a good site documenting freeway speeds in the US that range from 60 to 85 mph (96 to 137 kph). You can bet that people in many of these areas are driving 5 to 10 mph above the posted limit. Thus, "fast" will depend a bit on where you are from.

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

posted over 6 years ago

CC BY-SA 3.0

TimothyEbert‭

5 reputation 0 1 1 0

Many have mentioned that you might use km/hr as a writing tool to create an effect. I'd like to offer a global answer: if you're not intentionally using km/hr to create an effect, use mph.

If you use a number in writing, the reader has to interpret that. To do so, they will use things like context clues. Some SI units are decently easy for people to interpret. If I say someone struggled to lift a 20 kg mass, we can put 2 and 2 together pretty easily and get an intuitive feel for how heavy that object is.

km/hr, however, is a unit that most Americans are not very familiar with at all. For most of us, it's just "the little numbers on the inside of the speedometer that we always ignore." Myself, I work with SI units all the time by trade, and I still don't have a decent feel for km/hr. If you gave me 140 km/hr, I would be obliged to do the mental juggling required to convert it to 87 mph, and then I could compare it to highway speeds and get a sense of how fast it is.

If you want to use km/hr, make sure you give the reader strong hints as to how fast that is. Make sure the verbiage includes something like a reference to flying along at speeds that would get you pulled over on many highways. Or maybe it gets compared to the speed of a major league fastball.

On the other hand, if your text has one of the characters mentioning it's 70 km to a particular city in Italy, knowing that they are traveling at 140 km/hr does tell the reader that they have a 30 minute drive ahead of them. It doesn't give a sense of how fast they are driving, but it does give a sense of how long the characters have to relax before they get there.

posted over 6 years ago

CC BY-SA 3.0

Cort Ammon - Reinstate Monica‭

35 reputation 0 6 7 0