Hi I am a Brit Monty owner. The original speedo calibration is very accurate. I fitted a mph speedo to my lhd car. However nighttime driving is hopeless. Now with all the cameras and speed traps I have resorted to using a tomtom satnav. Essential for nighttime and you can change from mph to kph quite easily, but no furlongs! Not original I know but I no longer worry about speeding and enjoy the driving experience. Not sure on the US laws regarding speed detectors though.