You can get realtime fuel consumption somewhat easily -- this tells you how much fuel you're using but does not divide it by speed to tell you how far you go on that fuel. Getting realtime fuel economy (l/100km) may be more difficult.
Take a look at the DIY fuel rate meter link in my sig. In that post there's links to a couple other ideas too. It's useful for many (but not all) of the same purposes as a realtime fuel economy readout and a tells you a couple things that a fuel economy readout wouldn't.
Yup. Fuel rate easily tells you the difference between two options at a given speed -- for example, choosing which gear to use at 40mph. It also tells you when you've triggered DFCO. It's not quite as useful for deciding whether to go faster or slower since it just tells you fuel rate, not economy.
Specifically, it tells you the injector's duty cycle (percentage of time it's open). With a little bit of research and arithmetic you could make it into liters per hour, though that's not really a worthwhile way to use it; instead you just compare numbers on the meter.