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Source: https://writers.stackexchange.com/a/34775
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/34775
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

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.

Original score: 19