Paps, my Gauge is digital just to let you know. My car also have that efficiency meter but it is not accurate. It just provides you real time KM/L reading. The most accurate way is to measure trip meter when you had your previous full tank then your current full tank. Sir Drick is right.
Below is the Screenshot. I use my Trip Meter B which i set back to Zero with my previous full tank.
Kilometers Covered: 180.7KM
Gasonline: Vpower Nitro Racing (Since my MX was born till now)
Liters: 3.495
Efficiency: 51.702
All stock except on Tubeless Tires. 90/80 Back, 80/80 Front.