Incorrect speedo on XF

[85XF]

Hello there.
I have recently replaced an engine (the 3rd over 13 years) into my trusty XF pano. The new engine is an unleaded 4.1 with standard 4 speed.
The XF used to run leaded and I used to have the big cam and extractors and all that but I got sick & tired of the extractors melting everything, no matter how much I heat-wrapped nearly everything. Over the years I replaced many engine mounts, clutch cables, accelerator cables, air-con hoses etc. So when the last engine died; that was it. A new standard unleaded one went in.
Now: There must be some discontinuity between the leaded & unleaded engine/gearbox and the existing speedo.
The speedo reads slower than my actual speed AND it's exponential
ie: It gets worse the faster I go.
eg: Speedo reads 60 - I am travelling 70. Speedo reads 80 - I am travelling 95. And the best one: Speedo reads 100 - I am travelling 140km/h.
Not good.
Especially after last night when I was pinged for doing 150 on the M2. Lucky the copper was "cool", believed me and only got me for 15km/h over.
It's apparent that my excuse (above essay) is starting to wear thin. And for the fact that I'm over not knowing how fast I am actually going.
So guys: What do I replace? Is it a simple spigot/sprocket thing that joins the gearbox? Is it standard? etc etc.
Thanks in advance. And thanks for reading all this.

BIBS