Speed Demon Achievement
To get the Speed Demon Achievement you have to reach 240mph, which is easier said than done!
The recommended technique for this is to use the Bugatti Veyron (though the SSC Ultimate Aero or Koenigsegg Agera also work well) on Le Mans Old Mulsanne Circuit. This has a very long straight, and if you play it in Free Play mode you can simply cruise through the first few corners and then put the hammer down on the straight. You should be able to reach 240mph with relative ease.
If you do not have it already, you can get the Flat Out achievement at this point, which seems to be awarded for holding a very fast speed for a good few seconds.
Added by: MongooseGeneral Oct 29th 2011, ID#7918