Speedo after mag upgrade

[Faded_Mantis]

I've done a search for this but couldn't find anything.

If I was to upgrade my mags, say from the standard 15" 205/65/R15 to 16" 215/60/R16 then how much would my speedo over read by and can it be fixed? (like changing the speedo spring, or getting the ECu edited?)

Also if I went up to 16" mags, but used 205/55/R16 tyres, so that the radius is the same as the default tyres would the speedo give the same reading as it used to on the old tyres? if so this may be my best option.