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A 205/55 R16 tyre has a 205 mm section width, a sidewall 55% of that at 112.75 mm, and a 16 inch rim - giving an overall diameter of 631 mm. Rolling circumference of 1982 mm means 504 revolutions per kilometre, which is what the speedometer is calibrated against, so changing tyre size shifts the reading.
Sidewall height
height = section width x aspect ratio / 100
Overall diameter
diameter = rim(in) x 25.4 + 2 x sidewall height
Because the reading is derived from wheel revolutions. A larger rolling circumference covers more ground per turn, so the display reads low.
A larger rim with a lower aspect ratio, keeping overall diameter within about 3% so gearing and the speedometer stay correct.