Abbreviations

Q. Can you direct me to the reference in CMOS that would cover writing 15mph (no space) or 150 mph (with space) and 8mm (no space) or 8 mm (space)? Thank you!

A. That would be CMOS 10.55, which says there should be a space between a numeral and an abbreviation. More examples (including some exceptions) can be found at 9.18 and 10.64.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]