Trips not linked to a mileage type use the global rate which can be changed on the settings page.

Trips linked to a mileage type will use one of the rates setup for that mileage type where the date of the trip falls in between the start and the end of the rate.

If you need to track some trips that don't need a rate calculation then setup a mileage type, for example called personal, with no rates and link those trips to that mileage type.

If you linked a trip to a mileage type but the rate is not being calculated, confirm that you have a mileage rate with a start and end date that the date of the trip falls in between. For example if you only have a single rate with a start date of the first of last year and the end date is the end of last year, then trips for this year will not a have a calculation.