A calculator?... "i am doing 64.5 mph i am still at 111.12 miles from destination... hmm let me bring up the built in calculator to see how much time i still need".
With fuel prices on the rise again, it'd be great to track fuel efficiency against a ton of different variables--you could find out if you really do get better mileage by lifting off the pedal slightly when you hit your cruising speed, and what temperature and humidity do to your mileage, etc.