So if you uncheck "reset automatically" here on the Trip Computer screen you can use this screen to give you the information you're looking for...
If it's the hassle of getting to this screen you wish to avoid then move the idrive menu selection over Trip Computer on the previous screen and assign it to one of your presets (the number keys on the centre console... just hold down the number you want to assign to this function and wait for the confirmation chime) which is what I do to quickly view this info
of course if you want it per trip, leave the automatic reset option checked... personally I like to have the control