Find a dealer that will take a credit card for some or all of the total, and use this to get a zillion miles and other CC perks. And/or find a 0% interest card.
Even if you can only put part of it on a card, this seems the way to go. If it's not 0% (or super low), pay that off right away but get points and miles for it. Those benefits, if worked carefully, will likely be more than the difference in returns between ~~3% on your loan and whatever you'd get in the market.
The only time I've ever financed a vehicle, I put as much as they would allow on a card. (If memory serves, and it may not, I think it was $7500, limited by the dealer.) I wasn't savvy enough to open a new card and use this for my minimum spend, but I did get lots of points on my regular card. The rest I financed via USAA at 0.9%, IIRC. (This was roughly 10 years ago.) But I hated having a car loan and ended up just paying it un full after a few months, even though I knew it might not be the very best financial choice at those rates.