E46 320d vs E90 320d mpg

[scot_aln]

Does anyone have or has had both of these? We are currently finding the e90 to be about 15% down on the economy of the older car, same driver, same journey etc... is this normal or do we have a sick e90? Both have covered similar miles, about 100k each and both are manuals in fact even same wheel sizes. Ignoring the fuel computers since they tend to be a little optimistic and calculating mileage vs fuel used the E46 is doing just over 46mpg and the E90 just over 39mpg.