What I hate about these fuel mileage missions is if you let the car idle for long periods, you have to apply 50%+ throttle to re-engage the transmission for some reason.
I lost a good 1-2% on the Veyron mission cause of it,
Still got gold, but I can see it being possible to do close to a full lap, if you can get close to wipperman, it is pretty much downhill from that point on