I have seen people say both 0.25 mile and 0.25 miles. Should we use plural or singular for a fraction of a mile?
The rule I follow is that you use the singular only when talking of exactly one mile. You'd also say half a mile, quarter of a mile, etc., but in these cases it's as though you're still talking of "a mile" first, and then taking half or quarter of it.
Let me say something further, with the caveat that it may be my own idiosyncratic usage: use the singular only when talking of the natural number 1, that is, when you're essentially "counting" miles and there's a single one of them. Use the plural in all other cases, even for the real number 1.0: just like "0.9 miles" or "1.1 miles", also say "1.0 miles" (but "1 mile").
[Mathematically, natural numbers ("counting numbers") are a subset of real numbers, but more properly what we have is an inclusion map, so it does make sense to distinguish between 1 and 1.0, actually. And the distinction between natural numbers and real numbers does have applications elsewhere in English: consider "fewer than" versus "less than". You'd say "fewer than five items" but "less than 5.0 inches", since a length doesn't have to be an integer number of inches, you're not counting but assigning a real value.]
Another aside: the SI standard for symbols is to use the same symbol for singular or plural, so you'd write "2 km" even if you were reading it as "2 kilometres". Of course, since you're using miles, you probably don't care about this. :-)
For reasons I'm not too sure about, we usually say "0.25 miles" rather than "0.25 mile". If you Google for both terms, there are about 700,000 "0.25 miles" but less than 100,000 "0.25 mile", and the first few hits for "0.25 mile" give things like "0.25 mile swim" or "0.25 mile wireless setup", where the "mile" is forced to singular anyway.
I'd leave it singular unless talking about a range such as "0.25 - 2 miles". In a table, for example, the heading would be the same regardless of the units. You can always abbreviate as "mi" and avoid the issue.
Simon's answer is also good. Also note that it's actually more awkward to say 0.25 mile than "point two five of a mile".
If we followed the definition of "plural", the correct usage would be:
1.3 mile (or 1.3 miles, according to Wiktionary note below)
2.2 miles ..
As the definition is:
Plural: of, relating to, or constituting a class of grammatical forms usually used to denote more than one or in some languages more than two
However, as Shinto pointed out in his answer here, people usually use the plural form for 0.25.
Here is an interesting note from the Wiktionary about the usage of plural for fractions (and decimals) in general:
While the plural form generally refers to two or more persons or things, that is not always the case. The plural form is often used for zero persons or things, for fractional things in a quantity greater than one, and for people or things when the quantity is unknown.
This does support the usage of plural for 0.25 though, since it is less than one. But if that is how people use it, we definitely can't say that it is incorrect.