Depends on the profile of the tyres too.
By increasing the rim diameter the tyre profile will decrease, usually giving you the same rolling diameter.
So the speedo should be unaffected.

I went from stock to 17" rims but he profile was decreased to 45. Speedo was still accurate.
My second set of rims were also 17", but had tyres with a profile of 40.
According to the GPS when the speedo reads 100kmh, I am actually only doing 97kmh.

Also smaller profile sidewall will usually be stiffer, therefore ride may be a bit harsher.

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