|The best way to do this is to play with a friend. In multiplayer you can't die, so you can focus more on hitting the speed ramps instead of having to dodge enemy fire. Again, vehicle and wheel upgrades will NOT increase your speed.|
In the level select screen you can only select the levels you've completed in single-player, however if your friend has completed more levels, have him/her host and you can play those levels. Even if you haven't unlocked those levels the game will keep track of your time.