I had the same issue with Turbotax miscalculating the percent of business use to lower than 50%. I found the problem was that the months (less than full year) for special allocation in the car and truck expenses worksheet would erroniously adjust the percent of business use. Since section 179 is not based on the amount of time used during the year, a work- around in the worksheet was to adjust the date placed in service at the start of the year.
