What will happen is that they will calculate it on the contributions you make yourself as an autonomo.
Some autonomos make only the legal minimum contributions necessary, then hike them up to the maximum for the last 15 years before retirement age. From 50 onwards if you intend to retire at 65. This could be a bit dodgy now as it is very likely that the period of calculation for contributions will change to a 20 year period. So the period to look at is from 45 onwards.
The one good point is that being an autonomo you yourself can decide how much you want to contribute - it is not just based on your earnings. In theory you could earn a relatively low salary, but decide to make quite high contributions for a good pension. There is a maximum you can contribute each month and a maximum pension you can receive each month. Unless you are a fat cat politician!
However, it should be ´within reason´ - if you only declare your earnings as 100 euros each month and then make the maximum contributions it could redflag Hacienda!!!! So you would need to be prepared to be able to explain how you are able to do this without them digging around for black money.
I´m not sure what the maximum autonomo contributions are at the moment - perhaps around 400 euros per month for 2010?
Hope this helps.
Would like to say that I´m new to this website and have found that it´s a really friendly informative site!