But in kph, the speed of light (commonly denoted as C) is roughly 1.07 Billion, so 60 million kph is approximately 1/18 of C. The speed of light is more often converted to kilometers per second, where it clocks in just a little slower than 300k. If I had to guess, the intent was to describe a speed of roughly 1/10 C, but in miles, since C=~670 million mph?
Maybe it would be better to refer to your interstellar speeds as fractions or multiples of C (or even just say lightspeed), rather than using units intended for very different situations? That would both be easier for most readers to understand intuitively, and be less prone to accidental conversion errors.
Sorry to necro an old comment, but I couldn't help myself.
More to the point, I really enjoyed what you've created so far. Thinking seriously about becoming a patron now.