Thanks for all the advice! Yeah I haven't thought about insurance yet, but that's a good point. Rates are generally lower for new cars but I think the car would depreciate more than the interest-rate savings.

So as an example Pentagon Federal offers:

1.99% - 36mo

2.49% - 48mo

2.99% - 60mo

So if I compare the smallest and largest with $15000:

1.99% = $465 interest, $430/mo payments for 36 mos

2.99% = $1168 interest, $269/mo payments for 60 mos

The difference in payments is $161/mo for 36 mos, so if I save and invest the difference every month at 5% interest, I get a total of $6092 saved. However, I've still got 24 months to keep paying, so that's -$6456 remaining. Meanwhile, my $6092 grows to $6456 at 5% for 24 months. So in this scenario, I break-even.

Anyone interested in following my math? Am I making sense? Or does lowest rate always win?