i disagree with using a sales price of zero. you made a gift of an asset that you deducted for business. in effect yo sold it to your son for its fair market value.
the other solution given would mean you could buy vehicles (as many as you could afford) for your business expense them (short term use only for business) and then give it to your spouse, children, relatives whomever, without tax consequences. I believe the IRS would disagree.
