I used to be following the official BSFC chart for a 2.2L sohc subaru engine which suggested best efficiency load of 80-90% below 3000rpm.
The Test
vehicle: 2009 subaru impreza 2.5i sedan auto
Experiment: Perform an acceleration run from dead stop to 70km/h and observe overall MPG at the finish checkpoint. Overall distance traveled will be 400 meters. All runs are to be in closed loop mode with A/F ratio of >14. Start and finish checkpoints are at exactly the same spots for all runs. Weather and temperature are the same (rainy and 2'c or 35'f). cruise control will be used to hold speed once at 70km/h. Car is fully warmed up before Run A. On a separate experiment find the best engine load for short distance acceleration (ie. heavy traffic).
observations :
Run....Load.......Shifts.........Overall Consumption........
A........60%.......1800rpm.......
12.1L/100km
B........80%.......2300rpm.......
12.3L/100km
C......+90%......3000rpm.......
13.3L/100km
conclusion:
acceleration at 60% engine load provided the most efficient acceleration. Even still run B with 80% engine load provided almost the same efficiency. The gap in efficiency widened when engine load exceeded 80% and the torque converter became fully unlocked. The TCU shiftpoints also increased dramatically past 80% load but this might Not be the reason for the 10% increase in consumption. More experiments with fixed shift points and longer distances are needed to confirm this. for now i will be accelerating at ~70% engine load instead of 80-90%.
For heavy traffic / short distance acceleration i found that minimum engine load is best. so apply as little throttle as possible when stuck in slow moving traffic with lots of stop and go. if you are to stop in a short distance (ie. red light in 100 feet apply no throttle at all and let the car roll). any considerable engine load increases consumption for short distance acceleration.