If you have an old car, and you want to measure the fuel consumption to adjust your driver technique then check out this device. It is build around one Arduino board, off course the project is open hardware like any other Arduino project, the LCD is a simple two row alphanumeric display, nothing fancy about it. To power up, just tap into the battery, connect it anywhere you can find +12V.
The device only works on cars with injector based fuel distribution, by counting the pulses and the time which the injector valves are open, and using a calibration table it computes the fuel burned by the engine, the device has also a menu to access some settings, you can even display the CPU usage in %.
With a little work you could also integrated it into the dashboard, probably won’t increase your old cars value, but it will look good and its a great DIY project.
Fuel consumption calculator: [Link]