Tire sizing confusion

The front wheel of my bike measures out like a regular 20" wheel. It's outside diameter is 16.5". According to the bicycle tire/wheel sizing chart I found online that makes it a 20" rim. The rubber strip in the wheel for protecting the tube says 20 x 1.75/2.125
The tire I just took off from it says 20 X 1.25 on the side (here's where it gets confusing) but it's outside diameter is only about 18.5".
Everything I can find online says the "20" stands for the approximate outside diameter of the tire, so 18.5" OD shouldn't be possible. The old tire is an IRC Metro. It obviously fits my "20" inch rim (16.5" actual dia) and it's width is about 1.25".
At any rate, I am at a loss as to how to figure out what tire size I need to order to end up with a tire the same size as my old one. I can't order a 20 x 1.25 because the ID of the tire will be too big for my rim (been there, done that). I could order the size indicated on the rubber strip in the wheel (20 x 1.75) but that's wide, like a BMX bike. This would be a whole lot easier if the first number of the tire size was the wheel diameter.

 

Rule of thumb: tire size in fractions are a different bead diameter than those expressed in decimal. That said, it’s often best to shop for tires using the ETRTO bead diameter, sellers often don’t list tire and tube sizes correctly. I found this to be especially True for 16” wheels.

16x1.5. 305 ETRTO
16x1-3/8. 349 ETRTO
20x1.5. 406 ETRTO
20x1-3/8. 451 ETRTO
26x1.5. 559 ETRTO
26x1-3/8. ? ETRTO. Never needed that one.