I have a number of mathematical puzzles about a (ficticious) French friend of mine called Nepomuk. Here’s the first of a series:

Nepomuk is driving to Paris, and he passes a road sign telling him the distance left. Unluckily, he’s a bit short-sighted, and so all he sees on the sign is that the number of miles to Paris has three digits, with the middle one being a zero:

Nepomuk travels along at a constant speed, and after exactly one hour he passes a road sign telling him that he’s still YX miles from Paris — the same two digits appear, but inverted.

And yet another hour later (he’s still driving at the same constant speed), he sees a sign telling him that he’s only XY miles from Paris — the same digits as on the first sign, but without the zero.

What is the (constant) speed that Nepomuk is travelling at?