Q: New Englanders habitually mute or diminish the R phoneme (?) in many words, (park, car, Harvard, etc.). What is the name of this characteristic of their speech? So many of the patterns of New Englanders' speech (as distinct from many other regional US dialects) strikes me as a close mimic of the speech of our friends ‘across the pond,’ that I’m curious if the diminished R of BrE is noticeably distinct from that of NE AmE, and if so, where or amongst whom, in the UK is that distinction most pronounced?
English accents are commonly divided into two main groups: rhotic speakers pronounce a historical rhotic consonant (/r/) in all instances, whereas non-rhotic speakers pronounce /r/ only before or between vowels.
For example, a rhotic speaker pronounces words like hard and butter approximately as /ˈhɑrd/ and /ˈbʌtər/, whereas a non-rhotic speaker "drops" or "deletes" the r sound, pronouncing the words /ˈhɑːd/ and /ˈbʌtə/.
This link is fascinating on a big subject.
To expand on Hot Licks' observation of adding an "r" sound where there is none, some New Englanders (Boston) and Brits, while dropping the final "r" in words like mother and father, will, oddly, add one to a word that ends in a soft "a." During the Cuban Missile Crisis, JFK famously pronounced that country's name as "KYOO-ber." This habit is dying out in the US Northeast but appears to be going strong in Great Britain.