We are deducting the actual expenses of my wife's driving about 800 miles for employer-related work-- taking checks to the bank, picking up food for meetings, and so forth. I covered my carport, and added another shade structure for our cars, at a cost of around $30,000. Might I calculate her 800 miles of work-related driving, divided by her miles of total driving, times the cost of the new garage for that year-- like 1/50th of the cost (assuming the garage will last that long)? (Comes to about $37.)