Actually, it probably came with a 90/90-19 in 1978.
No, it came with a 3.25 x 19. Which would be an 82/82 - 19 in exact metric equivalent.
The 550's front rim has a 1.85 in. bead seat.
About fender-less...
Ever notice how similar emergency evasive maneuvers on the open road are just like maximum performance racing conditions?
Just when you are trying to eek out a near miss, the front forks twist and skew under load and a small road bump, and the front wheel twists a bit to go in an uncommanded direction. But then, chicks dig scars, slurred speech, disability checks, and wheel chair ramps.
...and crumpled up bikes with big tires on them.
Cheers,