use the gt one race car,set as u say,but set diff to 2.5.set gears to widest setting but then edit gear 5 and gear six manually to their limit,and hey presto the 2000 barrier is broken.im now up to 2219 kph.if u use this car and set up on 2 player mode,both using the same car,slipstream the two cars,i.e bumper to bumper and u can almost reach 3000. 
