The best thing to do is to calculate the kilometers a person travels during the month (you could base this on the average kilometers traveled by the employee based on their fuel reimbursement claims for the last six months) and fix liters of petrol or diesel as entitlement each month.
For example, if a person travels 100 kilometers per month, and their car gives, say, 15 kilometers per liter, then allow them 7 liters of fuel as their entitlement each month.
This will take care of frequent increases in the fuel rates and the need to revise the fuel reimbursement rates every now and then.
Regards,
Vasant Nair