Your example is backwards, but your point is valid. (When you increase the circumference of your tire, the speedometer reads slower than your actual speed.)
Here's another good calculator:
https://tiresize.com/comparison/
An example of particular interest to me:
ZL1 front: 285/30ZR20 Goodyear Eagle F1 Supercar 3
ZL1 rear: 305/30ZR20 Goodyear Eagle F1 Supercar 3
ZL1 1LE front: 305/30R19 Goodyear Eagle F1 Supercar 3R
ZL1 1LE rear: 325/30R19 Goodyear Eagle F1 Supercar 3R
There is about -2% difference between the ZL1 and ZL1 1LE tire sizes. So not only will the speedometer be off, for every 1,000 miles there's going to be an extra 20 miles on the odometer due to the error. It's not a lot, but I think it's enough to annoy me.
Also, the ratio front to rear is changed ever so slightly.
Zl1 Front vs Rear: 1.85529% difference
Zl1 1LE Front vs Rear: 1.89036% difference
Most likely it isn't going to make a lot of difference, but I'm still interested in knowing exactly what might be impacted.