Mileage

If when you drove those miles, there had a been at that time an agreement that they would reimburse you for the mileage, then to fail to do so is breach of contract: violation of the agreement, even if only an oral, or unwritten, one under which you drove for work in exchange for mileage reimbursement. So legally, if the agreement between you at the time had been that they pay for mileage, they should pay you. Unfortunately, practically, there is no good way to get the money if the don't voluntarily pay: the only way to involuntarily get it from them is to sue them. Suing your employer can be a drastic step, and may not be worth the impact on your career, or also the cost and time of the lawsuit, for the amout of money at  stake.
Note that there is no law requiring mileage reimbursement, and unless you have a written employment contract guarantying it, they can change their policy going forward: that is, they can decide to not provide reimbursement, and that new policy, while not applying to miles already driver, will be in effect for future miles from when they announce the policy to you forward. 
If either there are financial ("funds") problems at work, or if they don't honor their committments, this may not be a good place to work. You may wish to seek other employment. When you do get another job and are not dependent on them, at that time, you can decide whether to sue for the mileage they had owed you or not.