The Scanguage uses the SAE J1979 protocol. This standard as mandated by SAE for US vehicles has a variety of parameters. RPM, load, spark ign, LTFT, STFT to name a few. MPG is not one of them. So in order to compute MPG it has to send two messages through the OBD 2 port to request two readings: fuel level and distance the car has traveled since CEL has not illuminated.
The fuel level returns as a percentage. Maximum 100%. Minimum 0%. Meaning, if you have 9.2gal, OBD2 will show 100%. If you have 8.8gal, it will show 100%. But Scangauge computes based on what you input as the size of the tank.
Fuel level % = f
Tank size that you entered = T.
Distance traveled since CEL not illuminated = d.
MPG = d/(f*T)
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