It depends which defaults you've changed. If you've changed the settings on a part you've added in the 'Tuning' section, then you can revert to the defaults by removing the part, then refitting it - hey presto, the settings are back!
i.e. Full Customised Gearbox. You remove it at the 'Run and Setting' area, then put it back on. You'll find all the default settings have returned. Same goes for the suspension.
Yeah, you buy a Mustang Cobra, your dad takes a look in your garage just shakes his head and says, 'You could get much better mpg and cheaper insurance with a VW or Honda...'