Odometer Goals

In case you missed the backstory, here's the short version:
I'm building a high-precision odometer (aka "rally computer") as my first Arduino project.

So, aside from refreshing my embedded skills, and learning a fair amount about the Arduino environment, what should this project do?

• Provide two concurrent odometers - a 'trip' odometer, and an 'incremental' odometer.
• Provide a reasonably accurate (drift 1 second or less per day) clock.
• Provide an average speed display.
• Provide most configuration options through a run-time menu system, rather than only at compile-time.
• Maintain user configuration between power cycles.
• Run off a car's electrical system, with a backup battery as well.
• Be built entirely with dedicated parts - not everyone wants to have a game console controller to interact with their rally computer.

My initial version won't have all the features that some more advanced rally computers have - but it should do well enough for casual Time-Speed-Distance (TSD) rallying as well as Stage rally.