Recent versions includes universal method to calculate MPG (both instant and trip-average) on diesels. It may require some calibration, but for many modern diesels it works great. It doesn't require a specific 'fuel consumption' PID to be available on your vehicle. Just be OBD2 compatible.
I've used Dashcommand (Windows), Hobdrive (/hobd) and at least half a dozen other OBD SW and the most accurate MPG results for CRDI diesels were from /HOBD and Torque (android; paid version). BTW I drive a Hyundai Tucson CRDI. Have tried on a few other CRDI engines as well with good results.
PS: Don't have personal experience with Fiat MultiJet engines, but I've heard that Fiat Multijets are a tough nut (some custom protocol) and I'm yet to see a consumer grade OBD software which talks to MultiJet ECU's reliably. If nothing works, please have a look at http://www.fiatecuscan.net/