You said; "In short, i want to have records of everymonth when the petrol is filled in a particular car of a particular employee."
To have a record for each month, you have to create a NEW record for each month. Otherwise you are just storing the last months usage for a vehicle without any history. I don't see much value in that.
Since you have to enter a NEW record you need at least three pieces of info as I indicated. You need to enter the date so you know what period the record is for. You need to enter an amount so you know how much was used. Whether you only store the last period's usage or a record for each period you still have to enter those TWO pieces of info. The third piece, the Vehicle Code NEEDS to be in the record. If you are only storing the last period, then you are changing value on an existing record so the Code is already there. If you are using a subform to enter usage and the main form is bound to the Vehicle table linked on Vehicle code, then Access will automatically fill it in. But if you are just entering the data directly into a form bound to the usage table, then you need to enter the Vehicle Code.
To have a record for each month, you have to create a NEW record for each month. Otherwise you are just storing the last months usage for a vehicle without any history. I don't see much value in that.
Since you have to enter a NEW record you need at least three pieces of info as I indicated. You need to enter the date so you know what period the record is for. You need to enter an amount so you know how much was used. Whether you only store the last period's usage or a record for each period you still have to enter those TWO pieces of info. The third piece, the Vehicle Code NEEDS to be in the record. If you are only storing the last period, then you are changing value on an existing record so the Code is already there. If you are using a subform to enter usage and the main form is bound to the Vehicle table linked on Vehicle code, then Access will automatically fill it in. But if you are just entering the data directly into a form bound to the usage table, then you need to enter the Vehicle Code.