Put 20% down, finance for no more than 4 years, and keep total car costs under about 10% of gross income.
A popular US affordability heuristic. Long loans (72–84 months) feel cheap monthly but often leave you underwater when the car depreciates faster than the balance falls.
It was written for depreciating petrol cars bought on hire purchase. It says nothing sensible about a lease, where there is no equity to protect, and 20% down on a car that will hold its value is a different proposition from 20% on one that will not.
Estimate with the rule, then check it against the calculator that models it properly.
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