If you have the plastic diffs, tighten until the screw feels like its bottoming. Then back out half a turn. Take 2 flat screw drivers and insert them in the outdrive slots. If you can move the gear with the screwdrivers in there its too loose. With the car assembled if you hold down the front or rear wheels(which ever side the diff is on) and try to turn the spur, the spur should not move. Basically you want the differential action as free as possible without the gear slipping.