The phrase *Xkm* is plural in every instance, unless the value of *X* is exactly 1. We can see this from the fact that in speech, the word *kilometres*  (or *kilometers* for American readers) had the plural suffix, 'S' in cases apart from when preceded by the numeral *1*:

 - 1 kilometre
 - 0.5 kilometre**s**
 - 1.5 kilometre**s**
 - 1,000 kilomtre**s**

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). Is is because we can conceive of 2km as a single distance or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about *kilometre* noun phrases. We see this type of singular agreement with all types of plural measure phrases:

 - Twenty kilos is quite a lot to have to carry around all day.
 - Two tonnes is the absolute limit.
 - 100 decibels  is far too loud.
 - Three up-votes is not nearly enough.

Here is an Ngram which might give us an idea of the relative frequency of *kilometres is* and *kilometres are*. The blue line is *kilometres is*, the red *kilometres are*:

![][1]




[1]:https://books.google.com/ngrams/chart?content=kilometres+is%2C+kilometres+are&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Ckilometres%20is%3B%2Cc0%3B.t1%3B%2Ckilometres%20are%3B%2Cc0]