The smaller the vehical the easier it is to turn and they can fit through smaller areas, but their average speed is usually slower than the bigger vehicals. The larger the vehical, the faster they usually go, but they are harder to turn and can't fit through smaller areas.
The turbo will work faster, or accelerate the vehical faster if all 4 wheels are on the ground. If the vehical is air born, you will gain more air time than speed. This is good for jumps or ramping mud spots. Also, turboing on turns is a good way to keep a good speed, but can also make you lose control easier. Better to just wait and use the turbo on the straight aways instead of the turns, unless you are behind.
If you find yourself out of control, one of the best things you can do to get back under control without losing speed is to just let off the accelerator. Once you let off of the accelerator (the 'X' button), you can slowly stear yourself back under control. Hitting the brake may cause you to lose more control.
If you are having troubles knocking out 100 opponents to unlock the one truck racer, then go to circuit #7 (the one with the blue class one racer on it). Then select the race where you go against 5 other dirkbikes with a class one racer. All you have to do is run them over. My average for doing it was about 7 knock-outs!!