"Can I deduct all $15,000 which will increase my tax return by $15,000."
It doesn't work that way. A deduction is not the same as a tax credit. A deduction decreases the amount of taxable income subject to tax. The amount of tax reduction that results from that depends on your bracket as Super ee-ea mentioned above.
Also see the additional information I left you below.
Here's some more information for you:
Whether it is deductible depends on what you donated it to. i.e., it has to be an IRS-qualified charitable organization as shown here:
If the car was donated to an IRS-qualified charitable organization, this article at the link below tells how you can figure the amount you are allowed to deduct if you are able to itemize deductions on your return.
The deductible amount can also depend on what the charity was going to do with the car. Was the charity going to use the car in their activities, or was the charity going to auction the car and use the cash? If the charity was going to sell it, for instance, you can only deduct the amount the charity received (or expects to receive) when sold at auction, in most cases.
If the charity sold it, has the charity presented you your Form 1098-C yet?
Also see this IRS Publication 526 on charitable donations for more information about car donations: