the most important thing is that you buy only the car you need, not the car you want.
If you buy more than you need, the %'s don't matter, as you wasted 100% of the money you overspent.
That said, I don't know your income, job stability, or anything else, but try and keep the purchase price to 20% or less of your gross income. If you drive a lot, it may be worthwhile to pay for a slightly newer (NOT new) Honda civic, as they will last for years to come.
If you can get a good rate on a car loan which is lower than your student loans (the car which costs <20% of your gross income), then I'd say borrow as much as you can of the purchase price, pay the minimum on the car note, and keep paying down student loans as much as possible since the rate is so much higher.