Please read:

Motor A gets 6 more miles per gallon (than motor B). (1)
=Motor A gets 6 miles more per gallon (than motor B). (2)

Both are correct? If yes, what is the standard? Why? If not, why?What is the difference in their meanings?

Both fine and equivalent, as Inchoateknowledge said for your other, similar post.
And no need to write 'Please read', Quoc. It sounds odd; why else would we be looking at your post?
Teachers: We supply a list of EFL job vacancies
Dear teacher,

You wrote:

why else would we be looking at your post?

Please explain me the meaning of this sentence.

It is second conditional

It means, if it were not for reading it, then what other reason would there be to look at your letters you post in the fora.

In a nutshell, we read your letters, whether you ask us to do so or not.
They're the same in meaning.
Site Hint: Check out our list of pronunciation videos.

What is the difference in their meanings?

I'd like to add a small comment that changes in word order can cause small, often subtle, changes in emphasis. Beginner learners don't need to worry about this, but more advanced students should be aware of it.

Best wishes, Clive