It sounds like you want to keep a history of odometer readings versus date. Do you have something that uniquely identifies each vehicle (e.g. VIN, fleet number, etc.), and which you can use to link the two tables together?
For populating purchase date and mileage for the records that you already have...this can be accomplished a couple of ways. One is to use Query Design view in Access to build an insert query that will perform the record insertions into TB2. This is a pretty straightforward 'point and click' method to quickly accomplish what you want. But a unique identifier (primary key) is helpful...really almost necessary, to accomplish this. A more advanced way to do this is to write a SQL INSERT query. This is basically the same thing - and in fact when you use the design grid to build your query that's what Access is doing, and you can even switch to SQL view to see the SQL that Access constructed for you.
Going forward, I would eliminate the initial odometer reading column from TB1 and keep that information just in TB2, as this will eliminate duplication of the data. When you initially enter a vehicle in the database, you would want to populate both TB1 and TB2 with the necessary information, so that involves attaching TB2 to the form as well.
I highly suggest taking a look at the article in our Insights section on how to properly structure database tables:
Database Normalization and Table Structures. It has lots of useful information about creating primary keys and minimizing data duplication.
I know this is all pretty general, but I think it's the right direction for you to go in.
Pat