[duncanawoods]

>> But in common use I wouldn't be surprised if it was 98% or less

I would. One bit of evidence against this is the amount of the Extra's script Ricky Gervais had to rerecord for an American audience. Most of the change was for unshared cultural references (local food brands etc.) and only a couple for words (fanny).

Due to the level of exposure, I believe the difference between generic Brit and generic US is less than the dialects within the countries e.g. older generations of Yorkshire vs. Cornish, Glaswegian vs. Cockney etc. can suffer great difficulties understanding each other.

The number of unshared words is much higher, methods of denotation and sentence construction are different and even the pronunciation of shared words become unrecognisable (I'm not a linguist, so open to correction). I'm sure there are similar extremes in the US.

[jaimebuelta]

There's also the fact that, due AE being all the time in the TV, the average Brit can understand it much better than the other way around. A lot of americanisms are favoured by not-so-young people.

They were complains of American fans not understanding Peter Capaldi as the new Doctor, as he has a (VERY understandable) Scottish accent.

[duncanawoods]

I wonder to what extent American dialects are objectively easier to understand because they are the younger result of synthesising existing languages\accents rather than a UK dialect that has diverged due to isolation.

[iopq]

They're easier to understand because of Hollywood. If the United Kingdom made all the movies, everyone would think American dialects are really hard to understand.

Besides, nobody understands Southerners anyway.