Wow, you owe $49k on a 5 year 17.5% loan? That's over $8500 in annual interest right now, which equates to around 700 monthly interest.
If you typed the numbers right on what you need to refi, then yes, even if you paid a lot down, you are going to have negative equity or go into negative equity soon. Your payments are far too much interest vs principle. If you haven't yet developed a large negative equity, it's going to end up there, so I would suggest you do speak directly with a CU.
Personally, I would sell the car rather than pay that type of loan shark rate for a high value auto. Drive a junker for 2 years and just the interest savings nets you $17,000 which is 1/3 the cost of your entire car. Add the amortized principle you would have paid on top of that and you would have close to 1/2 the price of the car in cash.
Right now, IMO you can't afford the car. Even if you can make the payments, you are murdering your savings by not putting that in savings, but paying it to a finance company. I would downgrade asap unless you can get your APR down to 9% or less.
Don't make the finance company rich, start adding to your own wealth. Cars are wasting assets. And in this case you are paying 17.5% on the original value for an asset that is going down in value. The total cost to you is staggering. But again, this is just my opinion and offered as only that.
Good luck! 
Yes, you may need to put 20% down, which means on a 30k car you would only borrow 24k. Even if you received NO reduction in APR, the fact that you have a 24k balance as opposed to 49k saves you over $4k in interest just in the first year. And any reduction you could get in the APR (due to the lower loan size and percentage of down) means even more savings in your pocket.
$4-5k per year interest, just thrown away. That could max you annual IRA contributions. That could pay for a pretty nice vacation. That could buy a complete home entertainment center EVERY year. That could put $25k or more in your bank (not including any returns of interest paid to you) over the course of 5 years.
The point I was trying to illustrate is that you are actully GIVING UP a lot in order to drive the car and that car is depreciating in value and is aging toward costly repairs at some point.
Personally, I sell the car, put 20% down and buy the $25-30k car and be thrilled to see all that cash accumulate in my savings and investment or retirment accounts. The balance of the loan costs you interest. The cash in your account pays you interest.
Anyway, I hope it works out for you. My opinion, for what it is worth (or worthless ) is that the car is too expensive at that APR and you are throwing a ton of money away never to be recovered and to end up in a worthless asset so many years from now.