[wjy]

Many processors in a modern car have no floating point capability whatsoever. Fractional numbers are represented using integers - "fixed point" representation. While it's almost certainly true that vehicle speed is calculated as a fractional value, the cruise control set speed may be an integer, as it's set in increments of 1kph (or mph, depending). There's no need to keep smaller increments.

It all depends on which processor is handling the cruise control interface, and how they wrote the software. There's no way to know without more information.