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As others have said, many variables to consider...another is the engines output curve. As with many production cars, dyno graphs show power delivery is not always smooth with often dips / flat spots at certain rpms not to mention steep increases. Could be quite simply that the engine is at a more efficient power output rpm at 62 MPH cruise speed than at 55 MPH cruise speed. Since the speed difference is very small, this could be another answer to the question of why it is better at 7 MPH faster. A dyno of the car against speed might indeed show this. Probably the case for the other Camry results difference not being linear with speed increase.
I was bored and did a quick workout in excel based on aero drag alone, starting at 50 MPH and 10 HP required to maintain that speed right upto 100 MPH and the 80 HP (80.00003 as could not get it any closer than that) required to maintain that speed (as say just based on aero drag alone requiring 8 times the power to double the speed). So on my little mess about table I saw 12.31144 HP required to cruise at 50 MPH and 16.47182 HP for 62 MPH. So what if at that given rpm the engine is only producing 11 HP at 55 MPH and then a bit of a spike to 18 HP at 62 MPH. Just a thought. While aero drag can be plotted as it increases with the square of speed so you know exactly what it will be...engines outputs can not. While looking for that optimum run for best MPG, speed might have to be adjusted based on the specific engines power curve.
Ok I am waiting to be told off now.
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