High gas prices are causing many Americans to think about buying a more fuel efficient vehicle, but if you think you can donate your old car to charity and then take a hefty tax deduction, the American Institute of Certified Public Accountants advises you to brush up on the rules because they may be different than you remember."Taxpayers usually can't write off the full market value of a vehicle they donate to charity, but many taxpayers don't realize that the federal rules changed in 2005," Tom Ochsenschlager, vice president of taxation for the AICPA, said."Taxpayers have to meet strict