Originally Posted by xr2wishy

doesn't cost anything to do really.
you'll probably find they only read half the value from mfi to efi and vice versa, so just take clocks out and split open, under the rev counter bit there should be an adjustment screw with a dab of glue holding it in calibration, just pop this off and adjust with a screwdriver.
i don't have a pic at the moment sadly, but it's there for calibrating the multiplier of the signal to clock readout