you have to finish the lap with a fuel efficiency of 10 km/l or better. That means you can't use more than about 207 ccs of fuel, I think. It's not that hard to pass, you just have to avoid being a leadfoot. Let up on the gas about halfway through every straight, and you'll probably pass. Getting gold is another matter though...
I'd explain it more thouroughly, but I'm short on time at the moment...