Your only option is to consult a Fiat agency.

PiersCM said:

… yes, I've done that, to the letter - but no flashing…

Click to expand...

I might be able to clear this up, as its the same computer as the 2006 Suzuki Swift petrol I've got (they share a few dash components), short answer would be they added the average function at the facelift of the swift in 2008 until then it instaneous consumption only, have to assume they only added it about the same time to the sedici/sx4.

I enclose a picture of the display in my car so you can compare it. Looking at photos on Google actually you have a dash in its entirety right down to the steering wheel only differences are badge on wheel centre and the chrome trim round the gauges which is only on swift sports.

My petrol Sedici has the average mpg readout function, and it was first registered in Feb 2007, so probably built late 2006. Quite possible the early '06 models had different software, or maybe the OP's car was an import with different market spec.

I reckon that's it...
2006 different software...