20 mph or fewer?

[Flossbert]

I was telling a gripping driving-related anecdote earlier, about being stuck behind a car on an A road which continued to drive slowly as we approached the motorway.

"Even on the slip road he was doing less than 20 mph"

OR

"Even on the slip road he was doing fewer than 20 mph"

What say you, pedants?!

Less. Mph is continuous, you can go 7.5mph for example.

Is it as simple as that?! If you were to ask how many miles this numskull could cover in an hour, wouldn't the answer be "fewer than 20"?

No, because he could cover fewer than 19.5. Or 19.67834 if you fancy. Continuous data is less, discrete is fewer. There's a BBC article discussing the difference here.

Less.

Hmmm. Velly intelesting!! I shall consider myself told!

Personally I don't think discreteness or otherwise is anything to do with it- if you had four and a half apples and someone else had six, as a pedant you would, I hope, say you had "fewer" apples, not "less". However you might, if you were about to make pies with them, say that you were going (with your 4.5) to use "less apple". Apple is an aggregate quantity, whereas the apples themselves are items that can be counted, even using fractions.

So when you're talking about miles per hour, what you're really talking about is speed, which is an aggregate quantity, hence "20 mph or less" etc. As a pedant (or indeed as anyone) you could say that at 20mph you were doing 20 fewer miles per hour than someone doing 40, but your speed would still be "less" than theirs.

fewer with things you can count, less with things you can't.

less flour/fewer flowers.

Prism's got it covered.

I have to say, I had thought it was as prism says, although I could never articulate it so clearly, but I have heard that discreteness is the key, which is why I asked. I'm going to retreat to my original position to side with prism!
(I used to be indecisive, but now I'm not so sure!)

It is exactly as prism says. (and as I said, but with fewer words

You use less for things you can't count, like "speed" (generally speaking) in Prism's example and the idea of a mound of cooked apple rather than apples-as-single fruits.

I don't really understand the reference to discreteness though?

this link is better

oxforddictionaries.com/words/less-or-fewer

in everyday English if something is discrete then it can be counted

But it's the last paragraph that's important.

"Less is also used with numbers when they are on their own and with expressions of measurement or time"

Why thank you for that concurrence . I must admit that it's thoughts like these that led me to flirt with the idea of "10 items or less" being quite acceptable, as you could say that what it really means is "10 items or less shopping", not "10 or less items", and even a pedant could enter the express lane unscathed by crimes against grammar.

But this was only a flirtation.