You seem to be grasping the concept, but just to reinforce it, it's overall tire height.

If you put 225/50 16s on, you are changing diameter (height) by a bit over 1/4". Speedo error is less than 2%.

Not enough difference to overcome the error that is probably in the speedo already.

Most tire store sites have calculators to figure this sort of thing out.

 

Tire height = (tire width in mm x aspect ratio as a percentage) x 2 divided by 25.4 + rim diameter in inches. Now go play with your tire sizes.