Going from Tamil to Persian (Farsi) (فارسی) has a composite linguistic distance of 90% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Tamil → Arabic
Dravidian ↔ Indo-European
The 5 languages with the lowest linguistic distance from Tamil:
A quick 3-question check between Tamil and Persian (Farsi). No signup.