Hi I'm Billy and i will help you with this!
As these vehicles have problems with speed sensors, you need to check if the one you got was the updated one.
Having said that, i think your problem resides in the CAN bus (a network that allows car computers to communicate)
If it's a CAN bus problem, you need to take it to the dealer. At least get it scanned and get the problem pinned, after doing that i might be able to guide you through fixing it (if you wish to).
Looks like a visit to the dealer to get it scanned is the only option.
If it is the CAN bus what does this entail as regards XXXXX XXXXX and how much will as scan and possible repair cost.
Is a D.I.Y. repair possible if it is the CAN bus.
Speedometer repaired after 4 weeks at dealers. (use of courtesy car)
Scan did not turn up answer.
Inspection by Fiat manufacture technician found ABS control unit to be at fault.
Replaced unit with second hand unit. £60
all working ok.
Unit fitted for price of faulty scan diagnosis £60 no charge for courtesy car.