The fuel tax report, by default, records all mileage as on-highway mileage unless recorded within a geozone with the type of off highway.

Once mileage is accrued within a geozone of this type, you will begin to see it populate the fuel tax report with off highway mileage.

More information on geozones and how to create them can be found here:

What is a Geozone?

How to Create or Edit a Geozone