Great APRs for new and used car purchases through PenFed: https://www.penfed.org/index.asp
Even if you are not a military ID card holder or retiree you can open an account by making a one-time donation to the National Military Family Assn or Voices for America's troops.
If you qualify, you can get an APR for as low as 1.49% for up to 48 mths or 1.99% for up to 60 mths for a new or used car purchase.
Make sure to use their car buying website, which they will provide once you open an account. You can search for cars and dealers through it. You can even see a free Carfax report for that particular vehicle.
The site then provides a certificate, which will allow you to qualify for the rates above.
Potential drawback: No branch in CO.
PenFed