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I think you're right about "speed build up". Changing speeds on the GPS seem to be a bit jerky but cruising seems to be accurate.

UK law is based on EU standard, a speedo must never show less than the actual speed or more than 110% of actual speed +6.2mph so legally could read 50.25 at 40mph. Manufacturers calculate in to read high so they never read less when you take in rolling diameter changes to tyres from tyre pressures, wear and different manufacturers from OE tyres. Satellite navigation is not the device designated by which the vehicle speed is measured so none of those factors are relevant. Navigation is not very accurate at the build up speed but a lot closer when cruising at a constant.

Back to a good, trusted and recommended mechanic then, problem solved!

I've done some quick, non scientific googling on this and find nothing to substantiate that there is no deviation on a motorbike speedo. On the contrary, there seems to be a lot of sources suggesting the opposite. There does seem to be a lot of evidence, however, to suggest that GPS measurement is accurate, so I tend to think that my experience is probably correct.

In all the reviews that I have read since 2013 it has always been reported that the Speedo is very accurate with no deviation.

If my maths are Ok the sprocket etc would have to be vastly different as the suggestions so far would not give such large changes, my suggestion go to a trusted shop with a recommended mechanic.

When I ride with my GPS there is always a difference in speed between the bike's speedometer and the speed shown on the screen of my TomTom 450. I have assumed that the GPS is more likely to show an accurate speed (don't ask me why, just a gut feeling), which suggests that the speedometer overstates the speed by anywhere from 3-10kph, depending on how fast I am travelling. Top speed I have done is 130kph, yet there were still cars passing me, suggesting I was riding closer to the speed limit of 120kph. When I took a spin on my father in law's VFR 1200 there was the same gap between his speedom

Check you are on miles and not Km just a thought smcg in roasting Glasgow - makes a change

Dct check is easy , they did it the other way on me and no handbrake symbol on the dash when brake activated . So they must different. Check the panel for the symbol and I was referring to gearbox sprocket , I should have been clearer .