Speed sensor in the gearbox (it is on the 2.3L anyhow) - This sends signals (electronic pulses) to the ECU. Triumph know the HOMOLOGATED tyre diameter range (new to worn - approx 5mm tread depth - which is approx 15mm/rotation difference over tyre lifetime - say 0.5%) - and they obviously can calculate how many pulses per km/mile ensue - a clock chip for timing is a really cheap. Other mfrs use other sources: my KTM uses the pulses from the front ABS ring - So
@Steel would never run up mileage on the rearwheel. But tyre mfrs are allowed a rotational margin even of say a standard 240/50R16 - one might be 240/50.5 another 240/49.5. Not all mfrs publish the data.
Really GOOD GPS speed sensors are not cheap. SatNav sensors are usually pretty slow (5Hz was the norm) and not that precise (5-6 metres or more). A good speed sensor is 1 metre location accurate and will be at least 20Hz - many now are over 30Hz. I did investigate this stuff at depth about a decade ago - had some really good email exchanges with several chip and device mfrs.
Bear in mind that while GPS can be accurate you need clear access to sky. So GPS speedos do not and cannot work in tunnels and it is really easy to make even a good sensor read slow/fast with shielding on one side.
GPS is great for planes, boats and stuff like opencast mines. But crap in urban and alpine areas.