Speedometer gear ratio question

[Wanderer]

Hello fellow XS enthusiasts. I have a 1977 XS650, and I noticed that my speedometer seemed to be reading about 10 mph slower than actual speed at 60 mph. I verified this with the help of my phone's GPS. I tested the speedometer gear ratio and it turns out to be 2.75:1. What is the correct ratio for this model year? I guess I'll need a new speedometer gear in the correct ratio or my problem runs deeper and involves the speedometer itself. Were there variations in the ratios between different models? Perhaps the PO installed an incorrect part. Thanks in advance for your help. Mark

 

[pamcopete]

Well, all Japanese motorcycle speedometers are calibrated to show 60 MPH when the cable is spinning at 2240 rpm. With a front wheel diameter of 19" the rolling distance is about 79" per revolution, or 79/12 = 6.58 feet. At 60 MPH or 1 Mile Per Minute the wheel will rotate 5280/6.58 = 802 RPM. So, you would need a speedometer drive with a 2240/802 = 1:2.79 ratio which is pretty close to your 1:2.75

 

[Wanderer]

Thank you for your reply, If I understand correctly, the problem must be in the speedometer itself and not the gear being driven off of the front wheel.

 

[pamcopete]

Wanderer,

What size is your front wheel / tire?

 

[Wanderer]

Its a Battlax BT45 100/90/19 inflated to 32psi

 

[TwoManyXS1Bs]

Sounds like it's the speedo, then. Some of these bikes have experienced having the dial screws rattle loose, and speedo needles coming loose. Some of these instruments have been opened up for repair.

If the speed differential seems to be a constant 10 mph, the needle may need to be refitted.

If the dial face mount holes are elongated, the dial face may need to be repositioned.

If the speed differential is a constant percentage, the inner movement may need cleaning and servicing.

 

[Wanderer]

Any good tutorials for opening up the speedo and inspecting and cleaning the inner workings? Thanks, Mark

 

[TwoManyXS1Bs]

Yeah. If you do a forum search on 'speedometer', 'speedo', 'instrument', you'll find several threads. Also check 650Skull's album, he shows disassembly and reassembly technique. Also, in some of those threads is a link to an instrument repair facility...

 

[Wanderer]

Thanks for all your help....