I've been involved in Ingress almost since its release, so I have a lot of experience to draw upon. Unfortunately Niantic have never published details on the speed limits, and it's unlikely they ever will.
The limitation on game actions appears to be based on straight-line (well, shortest-geodesic) distance from your last action and the time between actions.
Imagine a circle expanding out from your last action, with the radius growing at 35mph (just over 55km/h). As long as your next action is within this circle, you'll be fine. There have been some reports that you get one "free" action if you've been under the speed limit for a while, but there's some debate about this.
There is also a maximum time that your actions are remembered for. Reports place this between 30 and 60 minutes. This feature allows you to hack a portal in city A, then fly to city B and still be able to hack virtually immediately, without having to wait for the 35mph limit to catch up to you.
Not all actions count towards the speed limit though. COMM is one example.
Having said all this, Niantic may have taken the time to implement something more complex by now.
Why 35mph? It's a common speed limit on a large number of residential roads in the US, particularly in California and around Mountain View.