Originally Posted by
David T
If you think the speedo is inaccurate the first step is to check the odometer against a measured mile. The longer the measured course the more accurate you can get. Here in the US it is convenient to use the mile markers on limited access highways. Once you know the odometer is accurate you check your speedometer. The math is easiest at 60 MPH because it works out to 60 seconds which equals 1 minute exactly. If the odometer is not accurate you probably have the wrong ratio in the angle drive. Got to fix that before you can check the speedometer. Once you have an accurate odometer, if the speedometer is not accurate it must be calibrated separately. One way is just to pull the needle off and stick it back on at 60 MPH while traveling at exactly 60 MPH. At least you will know it is accurate at that speed but can be way off at other speeds. To properly calibrate the speedometer it is removed and calibrated on a testing stand at 1,000 RPM's whcih is 60 MPH and then checked at other speeds. It can be off up to 10% but is supposed to be correct at 60 MPH. If the dust shield was slipping the needle would be bouncing so much you would not be able to read it. If it bounces a little it means one or both of the inner cables is slightly kinked and probably need lubrication. Pull the inner cables and put them on the floor on a clean newspaper. They should lay in a nice large, smooth circle with no kinks. Lube them with speedo lubricant (you can get it at an auto supply) and reinstall if they are not kinked.