Back in August 2011, I made this great fuel and distance calculator for my Nissan Pulsar. It worked by measuring the time the fuel injectors stay open. On the display you can read out the instantaneous fuel usage, average fuel usage (for the current tank of fuel), distance travelled in metres, speed, and other information too.
Unfortunately that car was stolen, and so I no longer have the device.
On the inside, it was an AVR microcontroller running at 16MHz. Extremely accurate timer routines measured the fuel injector delay in microseconds. From that, the amount of fuel in picolitres was calculated, and averages and totals were maintained using a custom arbitrary-precision math library.
The DIN enclosure was all wood, painted flat black to match the Pulsar's interior. It looked pretty good considering it was made by hand! I didn't have my cutting laser up and running back then...