According to Wikitionary, the american pronounciation of <utter> is [ˈʌɾɚ] in IPA, and <cara> in Spanish is pronounced as [ˈka.ɾa]. According to the IPA these are both the same voiced dental/alveolar/post-alveolar tap, but to me, a native Spanish (Spain) speaker and fluent English speaker, these two sounds are quite distinguishable, they do not sound the same: the first one sounds more like a <d> and the second one much more like a (tapped) <r>. To be more precise, when I pronounce both words and notice where my tongue taps, the first one is dental or alveolar and the second one is further back, post-alveolar.
Do others hear and/or feel the difference too, or am I delusional? If there is indeed a difference, why does the IPA not make a distinction (e.g. between dental and post-alveolar)? Or is there some diacritic that I can use to mark the difference? (If there is such a diacritic, I haven't seen it used ever; the standard transcriptions of these phones are as above.)
Furthermore, to me the <tt> of <utter> sounds the same as the <d> of <cada> in the fast speech of most (castilian) Spanish speakers. But apparently the standard way to transcribe the latter is ['ka.ða], with a voiced dental fricative. To me it both sounds and feels like (in terms of tongue positioning) as a voiced dental tap; the way it is pronounced doesn't give enough time for the supposed frication in [ð] to occur (although I can accept a [ð] if the word is said artificially slow). Now the lack of distinction between dental and post-alveolar voiced taps makes even less sense to me, because I would have to transcribe the Spanish phrase <cada cara> as [ˈka.ɾa ˈka.ɾa], contradicting the fact that the distinction between the <d> in <cada> and the <r> in <cara> is clearly phonemic (since these are different words).
An alternative title for this question (or perhaps a slightly different question) could be: Why does the IPA make no further distinction between dental, alveolar and post-alveolar within voiced taps?