Many people use the aftermarket tachs because they have the ability to accellerate the needle faster than the dampened stock ones. In some cars, probably not ours, the manufacturer puts a circuit on the tach to smooth it out, to slow the speed at which it reacts. This is so when it's at idle you don't see as much fluctuation and change. People were thinking that there was something wrong with their cars because the idles weren't rock stable so the factories dumbed them down. Some newer cars even have the computer controlling the tach, so it literally is being told by the computer what rpm to read no matter what the engine is actually doing.