In AP style, "mph" is written as "m.p.h." Always use the abbreviation with periods. There is no space between the letters and the periods. This abbreviation is used for miles per hour. It's generally preferred to spell out "miles per hour" on first reference, especially in longer pieces of writing, and then use the abbreviation "m.p.h." in subsequent references for brevity and consistency. However, if space is extremely limited or the context makes it clear, you can use "m.p.h." from the first instance. Avoid using "MPH" or "Mph" as these are not consistent with AP style. The use of periods is crucial to distinguish it from other possible interpretations.