Is the divorce agreement final and incorporated into the decree? Does it address the extra mileage charges? It sounds like it doesn't, in which case it will be argued that you are responsible for all charges related to the car, including the excess mileage.
Unless you thought of that issue before you inked the agreement, it's probably too late to change it, and in any case, if the agreement provides for your ex to get the car and you are only now considering the implications of the extra mileage charges and the agreement is silent, I don't see how you can "get the car back" just because she's not staying within the free mileage allowance.
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Better to ask the attorney who drafted this agreement.
(1) I may be guessing. Do not act or rely upon this info; (2) We have not established an attorney-client relationship; and (3) If you insist I tell you something upon which you can actually rely: don't eat yellow snow.
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