Topic: Speedometer Calibration

[RCS1956]

Hi, Happy Holidays. Has anyone here tried to re-calibrate their speedometers? Mine is off by about 15 MPH, and over the years, I've gotten used to knowing that when it says 60, I'm only going 45.  I've got stock tire and rim size, and the original cable.  The bike has 45,000 miles on it, but that may be wrong also since the speed is off. I searched the FAQ's but didn't find anything. My bikes a 1976 CB750k  I hope the tach is right, I'd hate to be red-lining and not know it!!!

[clarkjh]

have you check the speedo cable, it might be binding some place, or a kink in it.  would through the dial off.  Also if there was dirt inside the speedo could be effecting the reading as well.  Just my 2 cents

James

[Bodi]

Assuming you have the correct speedo and front wheel gear then the odometer should be correct (within reason). The odometer is gear driven and the speedo is driven by balancing a magnetic clutch against a clockspring - the gearing will be exact but the clutch/spring balance is not. Speedometers of this type are rarely (never?) exactly accurate but can be adjusted pretty close with a given tire size - police bikes for example have supposedly accurate speedos so that they can be used for giving speeding tickets and usually have "Calibrated" marked on the face.
The only way to change the speedo reading is by opening the case, this is a tricky job and will leave the closing ring distorted; there are a few instructions in these forums and on the web about how to do the opening and closing with minimal damage. The speedo assembly can be cleaned and lubed and there's a bit of adjustment available on the clockspring. Actually calibrating it is tough as you need a way to turn the input at a known RPM to use as a reference speed that you can adjust the needle reading to.

[ElCheapo]

I have a drill that turns at 30MPH exactly. I use this for calibrating my speedos and usually end up +/- 2MPH. You have to know the speed of your drill from a known good speedometer. But like the others has written. Your parts must be verified clean and lubed. You can not assume that any one part is clean unless you have cleaned it. This trick also works on Tachs too.

[RCS1956]

Thanks for the answers and info about the milage. Sounds like a lot of work. I'll just go with the flow of the traffic.....well, maybe a little faster.