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Lower average speed improves fuel economy, all else equal - though in the case of a car accelerating through fixed gear ratios, all else is not equal.
Probably the best metric to use is the average brake specific fuel consumption. I'll set up a hypothetical situation.
If (just as an example) peak BSFC happens at 75% load between 2000rpm and 3000rpm, and cruising on the highway will be at 2500rpm @ 50% load, you have a few ways to burn fuel:
1) Accelerate at 50% load, targeting 2500rpm as the midpoint between shifts. This will be slow acceleration that will taper as you approach your cruising speed. You'll have the same BSFC as cruising (minus torque converter losses) but none of it will be in the peak BSFC zone. Economy may be better just by virtue of lower average trip speed. The torque converter may also stay locked up even during acceleration.
2) Accelerate at 95% load at 3000-4000rpm up to cruising speed, then settle into cruising at 50% load / 2500rpm. This will be brisk acceleration and will maximize time spent cruising. At no point (other than transitionally) is the engine producing the most useful work. It shoots past peak BSFC during acceleration, then drops below it to cruise. Also, average trip speed will be higher. Although it will reduce the absolute time where the torque converter is locked up, more relative power will be sent through an unlocked torque converter.
3) Accelerate at 75% load targeting 2500rpm as the midpoint between shifts, then drop down to 50% load once cruising speed is achieved. To do this, you may need to back off the throttle to induce an early shift, as the computer may want to shift at a higher RPM due to the relatively high load. This technique targets staying in peak BSFC the most - essentially, the engine gets the most useful work out of each unit of fuel.
Generally speaking, #3 ought to be the most efficient. The only way it wouldn't be, is by increasing the average speed of the trip.
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