English uses two lexemes to denote that something is smaller in number or size/amount: "Less" and "fewer". "Less" is used for uncountable nouns ("I needed less time to mow the lawn today"), while "fewer" is used for countable ones ("I needed fewer attempts to get the lawnmower going today").
However, sometimes "less" is used with countable nouns (famously the "X items or less" register in supermarkets). When can countable nouns be used with "less"? Consider these examples:
- I'll only need five minutes or less to mow the lawn.
- A lawnmower may only go 5 mph or less.
- You may only use my lawnmower if you weigh 75 kg or less.
These sentences sound correct to me, while the use of "fewer" in these contexts seems incorrect or at least odd to me (as a non-native speaker of English):
- ? I'll only need five minutes or fewer to mow the lawn.
- ? A lawnmower may only go 5 mph or fewer.
- ? You may only use my lawnmower if you weigh 75 kg or fewer.
Which of these do native speakers prefer?
Could it be that "less" is acceptable, when the countable noun refers to an uncountable entity (such as time, speed or weight)? If so, is this restricted to units (minutes, mph, kg)? Also: Would this explain the use of "less" in "X items or less", as "items" as a countable noun refers to an abstract, uncountable entity (i.e. "shopping")?