The deduction you can take on your tax return is based on the fair market value of the car. You can start to determine that by visiting Kelley Blue Book or the NADA Guides, which both have excellent used car rating tools.
For example, if you determine that your car is worth $5,000, but you know it doesn't start because it needs a new battery, the fair market value is $5,000 minus the price of replacing the battery.
You can learn more about taxes and your donated car by visiting our Tax Deductions & Car Donations page.