I know this is an old thread, but I found it in my own Google searches on using Chaparral 2x to beat Le Mans. I just did it today.
Let me start by saying that I'm not a great driver. (Well, good enough to get through the game -- but usually based on overspending my way to victory.) Also, I have only begun to scratch the surface on tuning beyond the obvious upgrades.
And I've tried Le Mans probably ten times with about as many different cars, including the Rocket and DeltaWing and other cars that are out of their class but many use to win on the "technicality" of their efficiency. No luck for me.
But Chaparral 2x did the trick.
I read on another board about the Chappy needing some throttle for control. So, to maintain control, you have to keep some throttle open even while braking. I did that -- essentially pumping the throttle through those insane & wet turns.
I met the 750pp limitation by adding as much ballast as possible -- then only needing to reduce power to like 98.7%.
Second place was 14 seconds behind me. I picked up a handful of penalties along the way (a collision or two and a few shortcuts), but the Chappy is fast enough that it made up for my rather piss poor driving.
Glad to have ALL THE RACES behind me now ahead of Gran Turismo Sport launch date in... only twenty days now!
