wrong.
its all relative.
if you want the speedo to show the correct speed then:
the total distance from tire contact patch to the centre of wheel hub has to be the same whether you put 20" rims or 15" rims. thus, for increasing wheel dia, you have to compensate with thinner sidewall tires to keep the overall diameter the same, or else risk having a discrepancy in speedo readings.
Circumference values matter. Thats how gears work.