How many miles has it done?
/filter changed? When was air filter changed?
All of these can affect mpg.
55/60 mpg is very optimistic - perhaps if you drove at a constant 56mph.
Although 39 does seem a bit low.
We get approx 50 mpg with ours. That is mostly A roads with some town driving.
on an average motorway journey of approx 200 miles including town driving at each end we get approx 53mpg.
However, during winter months this drops by a couple of mpg due to lights,heaters, more strain on battery,winter diesel additives etc etc.
Bear in mind that driving at 80mph on the motorway will affect the consumption. Even slowing down to 75mph you will see a increasae of approx 2 or 3mpg averaged out over the journey.
Economy of diesels always drops in cold months - as the manufacturers put additives in to stop the diesel waxing ( not a problem in UK - but they still do it).
Once the spring arrives ( please hurry up) and the garages start stocking new stocks of diesel our economy improves a bit.