Why are you using a value list for your vehicle types. That would be a non-normalized database structure.
What you'd want is a table specifically for vehicle makes
Code:
tblVehicleMake
VM_ID VM_Description
1 Ford
2 Chevrolet
3 Chrysler
4 Toyota
Then store the VM_ID (which is an AUTONUMBER data type in tblVehicleMake) as a foreign key in your data entry table (the one where you're recording the mileage, etc). I suspect you have more problems with your data structure than anything else.
If you persist in using value lists that is a FORM DESIGN change, you would have to force an update to the ROW SOURCE of your combo boxes then save the form, otherwise all changes are temporary (until you close your form or re-load it)