My '18 Terrain is within two tenths higher than the hand calculated MPG. It seems to be linear with the amount of fuel purchased. A fill of a near empty tank will be towards the two tenths. A fill with a half empty tank will be about one tenth.
2018 Terrain Denali, 2.0T, Blue Steel Metalic/Black
2013 ATS Premium, 3.6 V6, White Diamond Tri-coat/Brownstone
1967 Camaro, 327, Butternut Yellow/Black