Why Does Fuel Economy Drop Over 40% of Max Speed?

You are using more power to maintain the higher speed than you would the lower speed.

More air resistance, more road resistance,

The same effect can be noticed on cars, good economy at 50mph rubbish economy at 70 mph same reason as above

AS nick name said drag is proportional to the square of velocity.

From wikipedia:-

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^{see derivation}

where

is the force of drag,
is the density of the fluid,^[3]
is the speed of the object relative to the fluid,
is the reference area,
is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car)

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In your case all the independent variables are constant apart from

If you look a little bit further down in the article you will see that:-

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Power

The power required to overcome the aerodynamic drag is given by:

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

Is the OP sure of his mileage figures?

Unless I've missed something here.....The consumption of 74km/litre looks mighty good to me. Over 300 miles/gallon! Even the lower figure of 53km/l comes to 242mpg! I want one of those. My 1990 250cc Kawasaki did about 60mpg and was good for its day.

There are some good answers here, but to be less technical (a first for me!) the more the throttle is opened the more fuel you will use, and you'll have to open the throttle more to go faster. Ultra simples!