Enough talk; it’s time for a quick and dirty experiment!
I did this one time on a level, straight road driving in the same direction with our GM car having an average gas mileage readout.
Zeroed MPG meter and trip odometer and then accelerated quickly from stopped to 55 mph; then read MPG at the 1/2 mile mark. Average MPG at 1/2 mile = 20.8
Repeat except the entire 1/2 mile was used to accelerate slowly to 55 mph. Average MPG at 1/2 mile = 24.1
It appears that over a longer distance, the MPG loss from quick acceleration will go away asymptotically.
On my last tank of gas, I purposely drove accelerating as gently as I could without being a major jerk to the rest of the traffic on the streets. I got 42.9 mpg on that tank of gas. My record best ever was almost 46 mpg achieved by accelerating “normally” but terminating my accelerations when I got to a cruising speed that was about 5 mph less than the speed limit of the highways I drove on.
The best thing about slow acceleration is that you will never get terrible gas mileage driving that way even though it may not be optimum, however, the gas mileage experts on the hypermiling forums claim that you can lower your gas mileage by accelerating too slowly.
One of these days, I may invest in a ScanGuage II and actually measure the amount of fuel burned while accelerating to a given cruising speed at different acceleration rates. Even if the amount of fuel burned is identical at different acceleration rates, the gas mileage during the acceleration will be lower at high acceleration simply because the car covered less distance during the acceleration.