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The 13th entry of "to" in OALD

used to show a quantity or rate

There are 2.54 centimetres to an inch.

This car does 30 miles to the gallon.

I'm wondering why the sencond example is used with the, not a.

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up vote 1 down vote accepted

"<This car> does 30 miles to the gallon" is an informal idiom. In this case, "the gallon" is not a specific gallon.

"<This car> gets 30 miles per gallon" is a more formal idiom with the same meaning.

"Given a gallon of gas, <this car> will go 30 miles" is a less common way of saying the same thing. In this case, the choice between "a" and "the" is based on whether you have a specific gallon of gas in mind. If someone just put a gallon of gas in the car, you would say "the gallon". If you are speaking about the car's gas mileage in general, you would say "a gallon".

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Oh, I see. I thought the gallon here was not specific. – Kinzle B Aug 5 '14 at 18:49
Actually, your impression was correct. I have clarified my answer, in response to your comment. – Jasper Aug 5 '14 at 18:50
I think "the gallon" refers, not to a specific gallon, but to a specific unit of measure. <This car> gets 30 miles per the "unit of measure known as a" gallon. – Jim Aug 6 '14 at 1:38
Can I say "there are 2.54 centimetres to the inch."? @Jim – Kinzle B Aug 6 '14 at 6:50
@ZhanlongZheng- Yes, nobody would have any problems with that usage, but "per inch" is probably more common. – Jim Aug 6 '14 at 14:40

There doesn't seem to be a consensus by usage, except that the is preferred to a.

Google Books Ngram showing higher counts for "to the gallon" vs. "to a gallon" Source: Google Books Ngram

Either one is acceptable, but searching for usage in context shows a different pattern. to the gallon is primarily associated with the ratio of miles to gallons:

The car does 30 miles to the gallon.

But to a gallon is primarily associated with addition or additives:

This adds 40 cents to a gallon of gas.

When in doubt, do what I did. Plug the concept into a search engine and see the context.

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Why not just look for “miles to the/a gallon?” The difference is even more striking. – Tyler James Young Aug 5 '14 at 21:35
"Miles per gallon" beats them both. – 200_success Aug 5 '14 at 22:22

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