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I imagine that the speedo drive will come from the gearbox (or perhaps the diff) so it is the rear tyres that will impact any change in the speedo accuracy.
There are tyre size calculator apps that will tell you the change in speedo accuracy. For example, a 245/35/18 to a 245/40/18 will mean the tyre diameter increases from 24.75" to 25.72". At 60mph the speedo will read 62.34mph.
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