So what you describe in 1. are three fields in the same record of the same table? Do you also have a field that uniquely identifies each vehicle?
If so, a sorted self join relationship can be devised that enables a calculation to access data for the record in the same table that logged the same vehicle's previous inspection.
Please see fields being used below.
The rest of your description has gone over my head I'm afraid, sorry
Apologies, my answer wasn't really intended to tell you all you needed to know to do this, just to indicate that it is possible provided you have a field that uniquely identifies each vehicle that you inspect.
But I recommend that you define a table of vehicles with an auto-entered serial number field where you store the VIN number for each vehicle plus any other data (year, model, make?) specific to that vehicle that you might need. You could then set up this relationship:
Vehicles::__pkVehicleID = PMI Converted::_fkVehicleID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
A portal on the vehicles layout, among many other advantages, could list al inspection records from PMI Converted for a given vehicle. (And I'd rename your table to something more descriptive than "PMI Converted". )
But that's an option for you to consider, the following method can use either Vehicle Reg or my suggested _fkVehicleID field to produce what you want.
So each place in the following example where you see _fkVehicleID, you can use Vehicle Reg if you prefer.
Open Manage | Database | Relationships.
Select PMI Converted by clicking it and then create a duplicate Tutorial: What are Table Occurrences? by clicking the duplicate button (two plus signs).
Drag from _fkVehicleID in one table occurrence box to _fkVehicleID in the other to link the two in a relationship matching records by VehicleID. Since these two occurrences both refer to the same table of records, this is called a "self join".
Double Click the relationships line to open the Edit Relationship dialog.
Click to select Date IN on both sides of the relationship, select the > operator from the drop down and click the Add button to produce:
PMI Converted::_fkVehicleID = PMI Converted 2::_fkVehicleID AND
PMI Converted::Date In > PMI Converted 2::Date In
Click the Sort button on the PMI Converted 2 side of the relationship and specify that the related records be sorted by Date In in descending order. This makes the most recent record with the same vehicle ID and a Date In less than the current record's the "first related record".
Click OK to close the Edit Relationship Dialog.
Click the FIelds tab and add a calcluation field to PMI Converted:
Date In - PMI Converted 2::Date Out
This will compute the number of days since the previous inspection. A similar calculation can give you the number of odometer miles since last inspection.