If your not doing B-spec remote races with the widget.
Then you earn most XP from "like the wind" races.
Once you unlock "Dream car" at level 24, that will be the Exp earner as you repeat the championship event over and over manualy.
Do the championship manualy untill you unlock the endurance races.
Do the Endurance races while out then do the dreamcar again when in front of PS3.
Then Repeat Indy 500 race.untill you unlock the Ring 4 hr race.
Then repeat Ring 4 hr race untill you unlock the Lemans 24hr
Then Repeat lemans 24hr untill you unlock Ring 24hr
if you cant leave the PS3 for 24hr then do the ring 4hr races.