Does a trade-in reduce sales tax in Hawaii? Yes — here's how much

How Hawaii treats a trade-in

Hawaii subtracts your trade-in from the purchase price before calculating tax, with no dollar cap. A $10,000 trade on a $40,000 car means you are taxed on $30,000.
The credit applies only when you are buying a NEW vehicle. On a used or certified pre-owned purchase, Hawaii taxes the full price and your trade does not reduce it — an easy and expensive thing to assume wrongly.
In numbers: at 4.712%, a $10,000 trade-in saves $471.20 on a new car — and nothing at all on a used one.

Manufacturer rebates are treated differently from trade-ins here: tax is calculated BEFORE the rebate comes off, so you pay tax on money you never kept.
A dealer's own discount does reduce the taxable amount. That distinction between a dealer discount and a factory rebate is worth understanding before you negotiate.

A trade-in lowers the amount taxed only when you buy a new car. On a used car the whole price is taxed, even with a trade-in.
A manufacturer rebate does not lower the tax: the dealer owes GET on the full price because the manufacturer pays the difference. A dealer's own discount does lower it.