A remarkable difference is that American English still keeps rhotacism, like old English. American English is called a "rhotic" variety as well as varieties in Irish, Canadian, Scottish English. British English drop the "r" except before vowels.  

Hi! Your question seems rooted in the writing/spelling system rather than in the differences between the two Englishes. American spelling may be slightly different, and Webster (who's behind it) did it mostly for political reasons (to distinguish Americans from the English, a move in support of independence), and made his recommendations often by removing so called "silent" letters  (e..g in colour) or reversing letters ( in metre).

However, in your question, the use of 26 letters in the alphabet to encode all the English sounds is the same in both RP and in Gen Am. This is because the inventory of consonants and vowels are the same in both varieties. The differences are in the phonotactic, not the inventory.

Hope this helps,

LH

Mainz, Germany

Dear Gomatam,

You may find the following summary account of American pronunciation of some use:

http://www.americanaccent.com/pronunciation.html

There is of course a good deal of region variation. Still there is also continuity--something called the North Germanic continuum, as I recall, extends through northern Germany, the Netherlands, the British Isles and across the Atlantic. (Someone speaking in a northern German dialect or regional pattern, sounds to an American ear, like German with a slight English accent.)

Accent and variations of accent do have some social significance in the U.S., but not quite the kind of social significance it often has in the U.K. Americans are more likely to respond, say, to how a person dresses than to the fine points of pronunciation. 

My experience has been that American English is often preferred for ESL, partly, perhaps, because it is easier to master?

H.G. Callaway

American English is preferred for ESL because it is more musical, hence from an intuitive point of view it seems easy (if you have the ear to hear it); and then because ESL environment is more likely to be influenced culture-wise (above all music and movies) by the US than any other English-speaking country.

I think that most non-native speakers adopt a mix (rarely only one variation prevails unless the person has been living for a long time in a given country), British English is always at least partly present in the mix for most (college-)educated non-native speakers for the simple reason that most English language education methods still take British English as a standard. 

A remarkable difference is that American English still keeps rhotacism, like old English. American English is called a "rhotic" variety as well as varieties in Irish, Canadian, Scottish English. British English drop the "r" except before vowels.  

Mainz, Germany

Dear Sindoni,

What you say about American English retaining its "r"s is generally quite correct; and this belongs to standard American speech, as on network TV and radio, say.

But there are exceptions to this generalization. "R" dropping is quite familiar in the U.S. because it belongs to New England (or at least Boston) speech. President Kennedy was a famous representative of this pattern, as I recall. Something similar extends down to New York City and Norther New Jersey. You get things like this:

"He was the driver of the car." comes out like,

He was the driva of the ca --with the final "a" is highly nasal.

This has been weakening in recent decades, however.

One way to think of this is that standards of British speech have long had an admixture of social prestige in the U.K. But the people who left to go to America were not usually those well established in high prestige positions in the U.K. (Some people had more reason to leave.) So, this separation of the speakers of American English drew more upon Northern English, Irish and other patterns to establish an alternative. There is at least one place in the U.S. where common speech among friends and familiars retains a somewhat Elizabethan character. Much the diversion between English and American English came about because British speech changed subsequent to the establishment of American patterns.

H.G. Callaway

It makes sense that human geography should have an important role in the formation of different variations of language across the USA, with Irish and Scottish contributing to the accents since they represented massive waves of old migration. 

Another reason for the melodic American English (that is my "lay'' definition of it) could be the relationship music-language, especially if we look at country music which is very American in its essence and it is a heritage of the Irish folklore to a great extent. American country music has evolved and now there are so many sub-genres but still it has retained its popularity throughout the country and beyond, and singing patters are still very particular - very open and bright sounding in terms of phrasing and language.

Petrol - gas, biscuit - cookie, holiday - vacation, post-mail, shop -store, taxi-cab

What exactly is your interest here? Are you interested in the fact how the graphematic representation corresponds to the sounds of both RP and GA? Or are you more interested in a phonetic overview of the different sounds of British and American varieties?

Of course, I am oversimplifying here, as there are many British varieties as there are many American varieties.

A first reference for you may be William Labov, who carried out phonetic analyses (and in my view, methodological sound ones) of different North American dialects. But as his focus was on sociolinguistic variables, writing is not taken into account.

Other than that I follow Lian-Hee Wee's explanantion, which is correct.

Daniela, you could not read the question properly. I mentioned Daniel JOnes. my question is whether there is any authentic pronunciation for American English? or as you said you follow william Labov and your view is Phonetic analyses by Labov is correct. But some other will follow another one and in their analysis their Phonetic analyses is correct. so What actually authentic phonetic analyses for both the Englishes. or to agree with  Lian-Hee Wee's explanation  

Dear Mohana, I did read your question properly, I am just a bit surprised that you call Daniel Jones' work a "study", it is a description. Therefore, I struggle to call this authentic as it is not data based. If your understanding does not require a data based (i.e. empirical!) method, I again follow Lian-Hee Wee, as he mentioned the Merriam's "A Pronouncing Dictionary of American English", written by Kenyon and Knott 1944 (or probably a newer edition). I am not sure, though, whether notes on the relationship between spellings and sounds are included.

Also, if I remember correctly, the 18th edition of Daniel Jones' Pronunciation Dictionary covers American English.

Thank you Daniela, for your clear perception. now it is very clear to me the difference. Keyon and Knott is opt and suitable to our Indian who are interested to go America for different perpose to follow the book for regular practice. 

Mainz, Germany

Dear Schroeder,

Thanks for your contributions here, which seem generally sound.

As for the "political" motivation of Webster's reforms of American spelling, these might also be regarded as more purely cultural. Certainly, it was important, especially in the early republic to be able to distinguish easily writings by Americans as contrasted with British writers.

It strikes me as parallel to the differences, say, between "standard German" in its German and Swiss varieties. They are pretty similar, though the differences tend to stand out. I am not sure I would say that the differences are primarily a political expression.

One question left unexamined is whether Webster's 19th-century reforms of American spelling actually facilitated something distinctive in general American pronunciation. But it strikes me that, e.g., "center" as contrasted with "centre" emphasizes the retention of the final "r." Right? The older British spellings are often hold-overs from Norman French or other French borrowings into English.

H.G. Callaway

Any standard textbook for phonetics published in the United States will describe 'General American' phonetics in the first two or three chapters. For example, you can read the books by Ladefoged, Shriberg & Kent, Small, Zsiga, and most introductions oto Phonology (Hayes, Nathan--that's me, Odden etc.) who will also introduce American phonetics. Most of the commentary above is correct, although someone mentions Canadian English as being r-less, which it is not. In that respect it mirrors Standard American.

Incidentally, Standard American is r-ful except for some New England varieties (especially around Boston), but also most Southern Dialects (Alabama, Texas etc.) and African American Vernacular English (AAVE, 'Black English'), which are also 'ah-less'.

Many (Western) American dialects also have lost the contrast between 'cot' and 'caught'. They are the same (roughly) west of the Mississippi and throughout all of Canada, although the Western American and Canadian vowels are different.