change tyres and no speedo error?

[kachnak]

Hello, i changed tyres from 225/45/R17 to 205/40/R17. Diameter of wheel is lower for cca 40mm.
But i do not see any changes in speedo on city radar. i always go 55 kph and there is 50 kph on radar before and after change tyres.
Is it possible?

 

[tubby-ben]

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[Markeyboy]

could be, if you had no tread on the larger tyres, and lots on the new ones it could be close to each other

other than that im stumped