Very simple after you consumed a tank of fuel the SGII will report what it thinks you have consumed. well this will be off for sure so now you take and adjust the SGII to match the same amount of fuel that you pumped in your tank to the SGII and that part is calibrated.
Now you take a GPS of any type and cruise along at 45, 55, 65, and 75 MPH and calibrate the SGII to match as closely as possible to the GPS. (NOT you stock speedometer!)
So now what is all this do??? Well it sets the stag for calculation fuel usage...
Mile Per Gallon Calucation
Speed (Miles Per Hour) / Fuel Usage (Gallons Per Hour) = Miles Per Gallon
Gallons Per Hour Calculation
Speed (Miles Per Hour) / Miles Per Gallon = Fuel Usage (Gallons Per Hour)
Speed Calculation
Fuel Usage (Gallons Per Hour) x Miles Per Gallon = Speed (Miles Per Hour)