Question: My car dealer says the loan is at a "flat rate of 8% per annum" for 3 years on a ₹3,00,000 loan, with equal annual instalments. My banker friend told me flat rate loans are always costlier than they sound. Roughly what would the equivalent reducing-balance (effective) annual interest rate be?
Answer: Approximately 12% per annum on a reducing-balance basis.

Supporting evidence:
title: Inline formula / method note
quote: Standard flat-to-reducing (effective) interest rate conversion formula — financial mathematics of instalment credit
context: Standard flat-to-reducing (effective) interest rate conversion formula — financial mathematics of instalment credit.