This discussion belongs to years ago so I hope that my comment will still be helpful First of all a necessary preamble consonants in Italian can be double or single as is well known however what is often overlooked in this description is that this contrast can only occur in certain positions ie in between vowels or between a vowel and a semivowel w and j or for plosives between a vowel and rl in all other position all consonants have an indeterminate length So in the word pasto meal neither the s nor the t can be described as being single or double they have an intermidiate indeterminate length pronouncing something like passto or pastto would be meaningless in Italian phonology unlike other languages featuring geminate consonants like Finnish So applying this concept to the pronunciation of r single r is pronounced as a flap double and indeterminate r are pronounced as a trill in standard pronounciation for example caro dear contains a flap while terra earth and treno train both contain a trill You can check this listening to the entries of the Dizionario di Ortografia e Pronuncia published by the Italian state television RAI and which is or used to be available on this page Using the trill in caro no matter how short would inevitably turn it into carro charriot  I am a native speaker Ive taught Italian to foreigners and I have studied and compared Italian phonology in depth Of course this claims are not sufficient for wikipedia but theres an inherent problem when referencing publications on the subject For long time phonology has been considered in Italy just a lesser part of grammar more importantly most study focussed on internal controversies and variations ie describing the language from within the language a more objective and scientific study of the phonetics is quite limited It suffices to say that even IPA has hardly ever been used in the description of Italian phonetics many authors using custom phonetic conventions even in promimenent works such as the one I linked before This means that while the contrast between e and  or  and o is widely discussed in many pubblication because theres a lot of variation in distribution little work can be found about other distinctive traits of Italian phonology that are uniform or nearly so among native speakers eg the assimilation of place of articulation of n with the following consonant the lengthening of the vowel in stressed open syllables inside words phonetic value of syllables like in pasto contrasting with the prescribed orthographic split pasto syllabletimed rythm etc The difference of distribution of flap vs trill for r as well as an accurate description of consonants of incontrastive length in general has been largely disreguarded