Tachometer for my bike

Thank you both for the quick responses.
I like the idea of removing the interval:
https://github.com/avanc/espruino-tachometer/blob/f0a45a866a77ee9ad4a05aedd50cc03f38c4974c/tachometer.js

Adding an e-paper is also an interesting idea. I have to see were to get one :-)

A general question on using Espruino:
Is it better (faster, less memory usage, ...) to initialise everything as I did in my example and then doing a save()?
Or writing an init function:

E.on('init', function() {
  // Initialise functions/variables and setWatch()
});