the overall gearing & speedo accuracy is all dependant on just the tyres rolling diameter.
long as the chosen tyres rolling dia matches the previous tyre, the gearing & speedo should work the same.
bigger alloys will shorten the sidewall profile for sharper steering response and allow the tyre makers to use softer grippy high speed compounds but may create a harsher ride quality depending on suspension setup.
note that more popular tyre sizes like 195/50/15 will be more affordable than less commonly used sizes.