There’s a well-known disagreement about how we pronounce the stem from which the adjective mayoral is derived, namely the noun mayor. In the UK (or in England, at least), we pronounce it as a homophone of mare. Both are monosyllabic, with the just SQUARE vowel, thus meə (or some might prefer to write mɛː). In the US, on the other hand, it is commonly disyllabic and rhymes with player, thus ˈmeɪɚ; though you do also get a monosyllabic variant mer (i.e. a homophone of mare), particularly when immediately followed by a proper name.
The OED offers a lengthy historical discussion of the pronunciation of the word. The nub is that
A disyllabic pronunciation existed in Middle English, where it was a variant of a more common monosyllabic one. … The disyllabic pronunciation survived in Britain at least into the 17th cent. … as one possible pronunciation, but other sources of similar date show that this was by then highly conservative in British usage. In North America, however, disyllabic pronunciations appear to have remained current in all periods.
I’ve sometimes wondered whether Mare Street in Hackney in northeast London ought really to be spelt Mayor Street.
Back to mayoral.
- In LPD I give the BrE mainpron as ˈmeərəl, with an altpron meɪˈɔːrəl. For AmE I give just ˈmeɪərəl. (With hindsight, I ought to have included AmE meɪˈɔːrəl too, perhaps as the AmE mainpron.)
- CPD/EPD, 18th edition, gives BrE ˈmeərəl, AmE ˈmeɪɔːrəl (sic), with no variants.
- OPD and the online OED give BrE ˈmɛːrəl, AmE meɪˈɔːrəl, ˈmeɪərəl.
- My main American reference dictionary, Webster’s Collegiate, 11th edition, gives three pronunciations, the equivalent in IPA of ˈmeɪərəl, ˈmerəl, meɪˈɔːrəl.
- Forvo has BrE ˈmeərəl, AmE meɪˈɔːrəl.
Time for a survey, perhaps; but even a preference poll isn’t going to reveal inconsistent usage like Alastair Stewart’s.