Reimbursement would cover normal tear/wear and gas. Therefore, as a standard practice, an employee cannot charge for these items additionally.
Typically, 'Mileage reimbursements' would be tracked under 'Travel, meals and entertainment' category in the SGA section of the P&L.
Part of proper internal controls, an employee expense report should be filled out by employee and approved by department manager noting trip, business purpose or project name, date, and mileage to calculate total.
Ususally you have a G/L# set up it goes to (if they pay by credit card- check your chart of accounts)(i.e. van expense/mileage expense- or look at a previous G/L posting that shows a "mileage transaction") You could use petty cash or create a check requisition to pay it back, but you need to see a previous - similar - transaction (credit card statement G/L breakdown) , However, the site to figure the mileage rate is IRS.gov and it was increased this year to .505/mile (please check with IRS to make sure)