How is the BJ8 speedometer recalibrated when the rear end has been replaced with 3:54 gear ratio. I know that it can be sent off and calibrated. What is the actual method used to recalibrate it. My son used his GPS when he was here from England and it appears I am about 8-9 mph low on the speedometer now. Would seem that a constant speed drill could determine a fixed speed (say 60 mph) which would correlate to the old ratio and then adjusted to read higher mph at the same drill rpm if there is a simple adjustment? Don't these things work off a magnetic coupled disk(eddy current) or is there an actual gear that must be changed?