We're talking 1.7 mpg which is 6% of the average...not a big difference. It's very difficult to measure fuel mileage accurately enough to make a comparison with that small difference. You have to drive exactly the same route at exactly the same speed, with exactly the same accelerations and decelerations, in exactly the same road/weather conditions on exactly the same fuel from the same fuel source.
That said, since the difference has occurred across different vehicles and difference instances of the trip, we can rule out differences in vehicles and weather.
What could make a difference?
1) A/C position inconsistent across all the trips, ie: not on going down and on coming back or on low going down and on high coming back.
2) Different kinds of elevation change, ie: 5 steep hills will use a lot more fuel then one long gentle incline.
3) Different driving conditions down and back, ie: you leave ATL at the same time each trip in rush hour traffic but you do not encounter traffic on the way back.
4) Fuel tank is not being filled in exactly the same manner with a difference on different directions of the trip, ie: one gas station you use going down is in an incline which keeps you from filling the tank full.
5) Window positions not consistent, ie: on the run down you have one window open but on the run back you have no windows open. Open windows can significantly affect aero.
....And so on and so forth
otfl
