The Gaslog and Garage are fully functional and are able to work together. This means when you put in gas mileage data into the gaslog, you can choose a car. When I have time tomorrow I will modify the garage so that it also displays fuel economy information about your vehicles (assuming you have gaslog information for that vehicle).
Anyway, the fun part.
You can edit previous gaslog entries and annotate which vehicle the gaslog entry was for. This will ensure a more accurate mileage when the garage module is fixed to display fuel economy data.