Vehicle Allowance Policy

Hello, 
I’m developing a vehicle allowance policy for our company and I’m curious if you have any examples. Our COO is looking (simply stated) for the policy to be a monthly lump sum plus gas card, while our controller is stating that it should be a mileage reimbursement. The goal is to have the allowance to be tax free to the employee. Please let me know if you have any experience in this area and I appreciate your support. 
Regards, 
Chris