Speedometer is off!

FallusMaximus

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Hi guys,

New here to posting on this forum, but a regular visitor.
My problem is that my speedometer is off and my brain just can't figure out why this is.

The former owner of my XS650 has swapped the front and rear and I put a custom mechanical speedo (60mph=2240 rpm / 60kmh =1400 rpm) on it.

My setup:

Front: GSX600F 17" front wheel WITH GSX600F Speedo drive pickup
Rear: GSX600f 17" rear wheel

Front sprocket : standard XS650 17T
Rear sprocket : standard XS650 34T

One would think that with the smaller suzuki front wheel the speedo would be off. But... it has the Speedo drive pickup from the suzuki as well. Shouldn't it show the correct speed now?

If I drive at 50 km/h my speedometer shows that I am driving 70 km/h or so.

Would love to fix this. Could somebody help me out?
 

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There's two basic kinds of speedometer errors:

Offset - Speedo shows a constant difference to true speed.
Ratio - Speedo is off by a constant percentange.
Combo - Both of the above

If you have an odometer in your speedo, check its readout against known distances. This would get you the 'ratio' error. You can also do this by attaching a piece of tape to the speedo cable, to act as a 'flag', rotate the wheel, watch the 'flag'. Knowing the wheel circumference, see if the 'flag' produces the proper number of turns. For a 60kmh = 1400 rpm ratio, that's 1000 meters = 1400 revolutions, or 1 meter = 1.4 revolutions.

Offset errors are corrected by needle positioning.

If the odometer shows negligible error, 'ratio' errors are corrected by repositioning of the needle's return spring base. Tricky job, best to replace the speedometer.

If the odometer shows an error, that would confirm a 'ratio' error. Three ways to address this:
Change the wheel's size or drive gearing. (Good luck finding that stuff)
Get a different speedometer.
Use a ratio corrector. An automotive item, little box with lots of little gears, attaches to speedo cable.
 
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I have a GPS app on my phone. It tells me that my Honda speedos are all off by ~10mph on the high end.
Which is fine 'cause that way I think I'm going faster than I really am! :D
1970's precision...
 
According to my calculations, your speedo is a 2:1 ratio. That's a Harley ratio.
The Oriental ratio is about 2.75:1, which is 40% faster, which gives you the 50 = 70 error.

Edit: Those are the speedo cable versus wheel turns ratios.
 
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