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With around town driving we generally only get about 460km per tank fill, whereas on an open road trip we got over 650km (and that was with the air con on and not driving very economically). Why would there be such a big difference between them.

Many Thanks

Jonathan

This is to be expected. When you are driving in town the car is often using fuel even when it isnt moving (ie; stopped at lights, in traffic etc). Also, there is vastly more fluctuation on the accelerator pedal, and every time you accelerate to move roughly 1.5 tonnes of metal up to speed, it chews the gas.
On the open road, the pedal is kept at much more of an even opening and the vehicle's own momentum helps to reduce fluctuating throttle opening and saves fuel.
Car manufacturers always quote the combined cycle fuel consumption, which is a mix of urban (city) and extra-urban (open road) running. However, the urban figure is significantly more than the extra-urban when you break it down.
So in summary, city running is always significantly more than open road running.