How big is the tank? Maybe it's smaller than you think. Or maybe its design is such that it gets an air pocket in it and it can't be filled to its intended capacity? (sounds daft, but who knows?)
You definitely need to run it down as low as you dare and then fill it until it is spilling out of the orifice - then check how many litres you have put in and see if it matches the claimed capacity.
And why does a Volvo definitely pull 6th gear better than a Ford with roughly the same platform and powertrain? Is the Volvo lower-geared overall? If so, might that be a reason for worse fuel consumption than the Ford?
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