Even pre-tax interest arbitrage opportunities are rare, never mind after-tax arbitrage. Think about it. If you can get 5% by depositing money in a bank, so can the dealer. If the dealer can get 5% by depositing money in a bank, why would they lend to you at a lower rate? The only possible reason is if they've made enough money on the car sale (or indirect money such as future service profits) to subsidize the arbitrage loss. It's fundamentally wrong to think that you can somehow beat the system. What's really happening is that you just aren't aware of how you are ultimately the one paying for your so-called profit.