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