There is no laid-down formula. It depends upon company policy. You can fix a rate (Rs. per kilometer) differently, based on the type of vehicles used (that is, 4-wheeler or 2-wheeler). But it should be ensured that the user of the vehicle maintains a log which gives a picture of Date/time, places visited, distance (kms driven), and the purpose of the visit. Calculate and pay/reimburse conveyance expenses on a weekly basis. For frequent users, you can also consider allowing a small imprest which would enable him to recoup, as and when necessary.
Thanks,
R K Nair