PaulS:
After all, why carry an additional display, when I already have a perfectly good one.
OK. That's fine. I presume, then, since you are using it as a speedometer that you know how to get data into the phone, and how to manipulate that data to produce the visual effects that you want. Adding more data, and more manipulation to produce more effects, should be trivial.
It's not that simple. I have an app that can do those things. It uses the GPS signal and calculates those values. So unfortunately I cannot take credit for that.
If you can recommend a nice weather proof display,
Is your phone weatherproof?
No, it isn't, but I don't leave my phone mounted to my bike, wherever I go. Whereas if I had a dedicated display I would mount that to the handlebar permanently.
And yes, when it starts to rain, I'll put my phone in my bag so it doesn't get wet.