The primary vehicle I drive is from US so we're in the exact opposite situation :). Everything is miles, and like you, I find the km portion to be too small.

We drive all over the place regularly (from Calgary to Dallas, and back, as well as across Canada), so I bought a cheap North America GPS ($99). I switch it from miles to km and vice-versa depending where I am, and it allows me to see exactly my speed without needing to look at the dash at all. it also warns me in a different colour if I'm going 5km+ over the limit, or 3m over, and even alerts on red-light cams.