That would appear to require two tables, Cars and GasUsage.
Set up this relationship:
Cars::__pkCarID = GasUsage::_fkCarID
A portal to GasUsage may be placed on the Cars layout for recording GasUsage for a given car. (You would have one record in Cars for each car for which you track mileage.)
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
When I pull _pkcarID over to _fkcarID then this always happens
it looks like that its always a many to many relation....
I suggest making a closer look at the thread for which I posted a link. Your __pkCarID field is not correctly defined. It needs a field option that will auto-enter unique values such as an auto-entered serial number.