In letters from his *T.C.B.S. friends Tolkien was called variously ‘Gabriel’, ‘Gab’, ‘Cludhari’ – nicknames whose origin is obscure and not mentioned in surviving correspondence – but mainly ‘John Ronald’, with isolated instances of ‘Ronald’ or ‘JRRT’. His few surviving letters to the T.C.B.S. are signed ‘John Ronald’. In a letter to *Joy Hill of 26 December 1971 he noted that his contemporaries used to write his initials as ‘JR(superscript2)T' and pronounce them ‘to rhyme with dirt’ (collection of Ren? van Rossenberg).
According to Humphrey Carpenter, when Tolkien ‘was an adult his intimates [presumably other than family] referred to him (as was customary at the time) by his surname, or called him “Tollers”, a hearty nickname typical of the period. To those not so close, especially in his later years, he was often known as “J.R.R.T.”’ (Biography, p. 13).
-From The J. R. R. Tolkien Companion and Guide: Volume 3: Reader’s Guide PART 2.
(Very highly recommended)