How do you get 3 star rankings in Mario kart wii?

To get 3 Stars in Mario Cart Wii you have to win all races ahead by at least 10-15 seconds... It doesnt matter how many times you fall or get hit... Just win all races by 10-15 seconds ahead of everyone.

I don't believe this is true. I have 3 stars on all but two of the 50 cc races, half of the 100 cc's and 2 of the 150 cc races (I'm planning on getting 3 stars in everything if I can) and in some of those cases, I only won by a margin of 4 seconds.

I believe it's a combination of things and to be honest, I'd love to know because I just completed one of the 50cc races that I only have two stars on. In 3 of 4 races, the margin between me and second place was 10+ seconds and 20+ seconds in the last race, yet, I didn't get 3 stars.........go figure!

If anybody knows exactly what the requirements are for 3 stars, please post it in here!