SBK 2011 FIM Superbike World Championship

"A flash in the rain

"Reach 300 km/h (186.41 mph) in a rainy Single Player Race."

I did this using the Yamaha World Superbike Team YZF R1 on Monza. Make sure you set the weather to 'rainy'.

The final straight (in other words, do almost a whole lap, ensuring you are going as fast as possible as you leave the final sweeping corner) should let you reach 300km/h, though you might not be able to slow down in time for the chicane once the achievement pops!