To the extent that mileage and expense amounts are paid as reimbursement for money expended by the employee, in the course and scope of employment, they are not considered to be wages. Any amount agreed upon, which is over and above that actually expended by the employee, and which results in taxable income to the employee, is considered part of regular wages and is enforceable as such. This includes "vehicle allowances" and other types of flat-rate expense payments.
Okla. Admin. Code § 380:30-1-10