Speed indicator 10 km/h too high

Good day I’m new here, come from Switzerland Canton of Bern. I bought the Suzuki 2 WD automatic machine in 2016. Currently km stand 36000 km I use a Garmin Navi. In March of this year I noticed that the speedometer always shows 10 km/h too much compared to the Navi. I am sure that this difference suddenly occurred. In my opinion, it was only about 5 km/h before. The original summer tyres are still mounted, so it can’t be related to the tyre size. I’ve already used another navigator, with the same result. I asked my Garagist if he could sort of correct that again. He said no, not possible. I can’t imagine that. Do you have an idea what you could do there. I think that’s stupid if the counter indicates 60 and I actually drive 50. Thanks for every tip. Beat