my oversized tacho aint workin properly

hey guys i have just installed a auto meter monster tach in my 240 and i believe it is under reading but not by a constant amount at idle it is right but by about 2000 rpm its reading 1500

and by 8500 its only reading like 6500 has any one had this sort of problem before i have made sure the right wires are connected at the back and it is getting a solid 12 volts power and i am running a wire directly from the coil so if any one has any ideas would be much appreciated

doug