Why do these two tires have completely different sizing types?

One's imperial sizing, the other metric. So to get the same width, convert from millimetres (120? 130? mm) to inches, by dividing by 25.4 and order that width... The rim sizes are all imperial, so no need to worry about getting an 18mm rim... :thumbsup:

Not quite as simple as that. 4.00-18 is an older inch size standard. 4 inches wide (more or less) and 100 aspect ratio (sidewall height) for 18 inch rim. A 4.10-18 is a low profile tire less than 4 inches wide, aspect ratio like 80 or 90 (sidewall height % of width) for an 18 inch rim.

So a 120/90-18 is probably close to a 5.10-18 both low profile tires.

Not quite as simple as that. 4.00-18 is an older inch size standard. 4 inches wide (more or less) and 100 aspect ratio (sidewall height) for 18 inch rim. A 4.10-18 is a low profile tire less than 4 inches wide, aspect ratio like 80 or 90 (sidewall height % of width) for an 18 inch rim.

So a 120/90-18 is probably close to a 5.10-18 both low profile tires.

Cool, thanks. And what about the front tire?

Stock TW = 80/100-21.

80/100-21 is 80mm wide, 100% aspect ratio, 21" rim. Crosses to a 3.00 - 21.

You can use 3.00-21, 3.50.21, 80/100-21, or lower profile 90/90-21

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