85% of Turkic by speakers is Turkish (Turkey), Azeri (Azerbaijan), Uzbek (Uzbekistan), Kazakh (Kazakhstan), and Uyghur [ʔʊjˈʁʊː]* (Xinjiang, China).
Assuming an urheimat stretching approx. from the Uyghur to the Sakha regions, the above distribution includes the southern portion of that, with the addition of spreading quite successfully west to and across the Caspian Sea.
You can see the other 15%, extending over a much large geographic distribution, with the maps on the Turkic languages wiki page (which is where these statistics came from).
There’s not a huge difference from Proto-Turkic to modern Turkish. Just some vowel changes and a voicing change on some consonants. And loss of /g/ as indicated by the silence of ‹ğ›. That’s most of it (of course there are a few more). https://en.wikipedia.org/wiki/Turkic_languages
Relatively high mutual intelligibility (for Oghuz branch). It’s not automatic, but it’s rather easy for speakers to adapt and acquire. For a full report, check out Beyond Highbrow’s article on the topic.
For more reference, internet answers, and anecdotes on mutual intelligibility here are some further links:
Word Reference forums
Victor Mair’s Mutual Intelligibility Quiz elicits some useful reports from readers
Odd source, but the limited examples are interesting
* How to Pronounce Uyghur
The native pronunciation of Uyghur is [ʔʊjˈʁʊː]/[ʔʊjˈʁʊɾ]. I’m not sure if the /r/ would be pronounced word-finally. For a little bit easier version of the correct pronunciation, you might say [ʊiˈʁu]/[ʊiˈʁuɾ]. For a negotiated, Anglicized pronunciation you’d be looking at options or tokens like these: [u.iˈɹu]/[u.iˈɹɚ]; [ˈu.iˌɹu]/[ˈu.iˌɹɚ], [ˈu.iˌɹʌ], [ˈu.iˌɹə] (the [i] could also be stressed, but this would be the least correct of the negotiated pronunciations); [wiˈɹu], [ˈwi.ɹu]/[ˈwi.ɹɚ], [ˈwi.uɚ].