Standard IRS reimbursement rate per mile driven is 67¢ per mile this year, which is essentially the per-mile average cost for driving a car. But like, with this sort of thing everyone has their own personal calculus for what they want to optimize for. Do they want to save as much money as possible? Do they want to have fun while shopping? Do they want to shop as quickly as they can? A lot of people will balance these priorities against each other and come up with a solution that isn't optimal in any one specific area.