Sri Lanka: the duty is in the price, the interest is the problem
Most of what a Sri Lankan car costs is import duty, and it is already reflected in the price you see, there is no separate duty field, and the price you enter is the duty-inclusive one. What is added on top is smaller: registration and revenue licence charges, which this page opens at 5% of the price as a working figure.
There is no trade-in credit, so a part-exchange lowers the amount financed by its equity and nothing else.
The figure that deserves the attention here is not the tax but the rate. Sri Lankan vehicle loans are advertised well into the teens, and this page opens at 14%, against 3% in Japan and 5% in France. At that rate the term choice stops being a matter of convenience: over five years the interest alone comes to roughly 40% of what you borrow, and stretching the loan to seven years to make the monthly figure look manageable adds a great deal more. Enter the largest down payment you can actually make and compare the total interest line across two or three terms before looking at the monthly payment at all.