Here's a pair of links to 2 different "gauges" to measure fuel usage divided by distance be it miles per gallon, kilometer per liter, or some other reference.
The first one is for Geoff Grahams 2 designs
Geoff's Projects - GPS Car Computer and
Geoff's Projects - GPS Boat Computer. The second is for Karl Grabes
PIC Microcontroller Projects . Both of them use the PIC Microchip as the CPU. I haven't built either one of them, and of course YMMV
Bill Higdon