Ok let me clear a bit. In arcade mode you have two racing options: Time Trials (10 of them) and Arcade races. Both will get you some %, so without completing them you can't reach 100%.
Time Trial will beside the % get you Panoz Esperante GTR-1 in your garage in Simulation mode if you win all 10 Trials. Only thing I am not sure here is that: do you get % completion if you place in 2nd or 3rd in time trial too, or do you have to win to get %? Because to get the Panoz you must win 'em all.
Arcade racing will get you % for winning and will unlock more tracks that become available in arcade mode and a few more cars. And you have to win at least 30 or so races in easy mode to unlock all tracks and you have to win all races in all modes to get access to all cars in arcade mode. You have a completion status drawing in the menu which shows your progress. The only good thing (or a legal cheat) is, that if you race in hard mode, you automatically get awarded wins for easy and medium mode and don't need to race them. Similar goes for medium mode - you get easy mode awarded, but not the hard one.
All wins in arcade and time trials will get you arround 30.5% of game completion (I know that for sure, because I got all gold license tests, won all time trials and all arcade races before even starting the simulation mode and that was the % shown in game).
EDIT: just corrected a few grammatical errors.