Speedo not reading above 20mph

[Magic Beans]

I've got a Rev 2 turbo that's had a Rev3+ injectors, ECU, loom etc conversion and the speedo isn't reading properly. It will register movement depending on your actual, going up to around 20mph, but that's about it.

I'm told the clocks/cluster was changed previously, but that didn't rectify the issue, so is the remaining logical cause the speed drive? The one it has is electrical, which I believe would register an engine warning light if it has failed. Could it be the gear has worn down, or...?

Any suggestions/tests welcomed, before I start ordering replacement parts.

[Magic Beans]

Update:

The whole speed sensor is out. The gear looks good, no obvious wear or damage. We've compared the odo reading now to back when the car was MOT'd and while it has changed, it's way off the true mileage it has covered.

It looks like both the speedo and odo are reading low, but what would cause this? Failure of the sensor, issues with the clocks or something else?