Tire size

Tire sizing can be complicated converting between classic tire sizes like 16 x 3.0 to modern 120/60 R17 nomenclature. This explains the logic and some of the reasoning behind it.

Treaditional tire sizing (pun intended) assumed tires that were round, like a donut, and had equivalent width and height. Modern tires and car tires have radically different widths and heights, like a very wide rim and shallow low-profile, so modern tire sizes incorporate that detail also.

Modern tire sizing the rim number obviously is the rim size in inches, ex: 17 inch. Often it's listed after the other two numbers with an "R" prefix - like "100/70 R17" . While slightly confusing, rim width is in mm and rim diameter in inches, but conversion is simple: 1 inch = 25.4mm. Modern tire sizing

If tire size is 100/70 R17 for example: first # is the width in mm (100) and 2nd is aspect ratio - aspect ratio gives height as percent of width (70 %). 100/70 R17 would have 17 inch rim, 100 mm wide sidewall to sidewall, and 70% of 100mm = 70mm tall height.

Traditional numbered tires like 17 x 2.25 assume an equal width and height (so aspect ratio = 100) given as a single decimal inch value (2.25 inches).

2.25 inches is about 60mm so actual tire size of a 17x2.25 in new format would be 60/100 R17

That said, the effective height of a 100/70 is 70mm and a 2.25 tube is 60, it's totally well close enough to fit in either.

That can help you find tires that fit. the numbers aren't always exact, but if you've for 3 inches clearance you wont fit a 120/100 tire. Another caveat, most modern-numbered tires are tubeless, this means they've gotta seat much tighter to the rim and as a result can be very very difficult to install yourself.

this is one of my favorite calculators online as it offers a visual and numerical comparison, but you have to convert the decimal inch values to modern format: https://www.tacomaworld.com/tirecalc?tires=90-90r10-130-90r10