Speedo Inaccuracy Explained (mostly)

The tyres I plan on using are 185/65R13. With the Vogue's 3.89:1 differential ratio, that works out to be 41.48km/h at 1500 RPM. In the speedo software I divide motor RPM by 36 (41.67 from 1500 RPM) and round up to the nearest integer so it should show 42km/h.
I placed the Vogue on axle stands last light and locked the controller's maximum motor speed to 1500 RPM.

I set the PC to monitor various controller variables including actual motor RPM.
Flooring the accelerator gave me 1499.9 RPM on the PC monitor and 42km/h on the speedo in the car.

So all the calibration and calculations are actually correct - so why does it show 67km/h when we are doing 60km/h?

About the only thing left is the diff ratio - but that can't be wrong - can it?
Historically, the Mk3 Vogue sedan was shipped with a 3.89:1 diff. The earlier Mk2 has a 4.22:1 then later in the run changed to 3.89:1 - or so the rumors say.

The 3.89 to 4.22 would make about an 8% difference to speedo ready. My 67 to 60 is around 10 to 11% - but it's close. The current tyres on the car are 175/70R13 which are ever-so-slightly larger than the ones I calibrated for but it's not much (this should slightly counter the different diff.).

Back to diff. ratio. When I bought the Vogue it had been built from three donor cars. We know one of them was a Vogue Sports because I have the boot lid and badge, BUT, the Humber Vogue Sports never shipped as a Mk3 in Australia, so the boot lid is from a Mk2 - might the diff. be as well?
(The Sports did not sell as a Humber in Australia because all Mk3 Humber Vogues were upgraded to the Sports Alloy Head Rapier spec. motor - no need for a Sports.)

I'm very happy with a 4.22:1 final drive ratio - I had even considered chasing one up so that's good.
So a speedo software change is in the works.