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Speedometer Tips
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Speedometer Adjustment

I don't mind some speedometer error but 7% or 8 % is excessive, so I decided to do something about it. Since other board members have reported similar errors, replacing the unit would not help. The GL1800 has an electronic input to the speedometer (no cable). The sensor is located on the right side near the exhaust header. The output are 5 volt pulses that vary in frequency with speed. My first though was to design a device to change the frequency of these pulses with relationship to the input. An ideal job for a frequency to voltage to frequency converter. I was well along my way when I discovered there were commercial devices available. After a few hours of measuring the output of the GL1800 speed sensor and working with Dakota Digital http://www.dakotadigital.com/ Technical help (thanks David!), I determined either the model SGI-5 ($79) or SGI-6 ($110) would work. I bought both of them. The SGI-5 is a multipurpose device for changing the speedometer speed and the SGI- 6 is specifically designed for Harley Davidson applications. Both of these devices can vary speedometer correction .502 (1/2) to 2.0 (double) in 256 steps. The SGI-5 does it via a set of 8 dipswitches and the SGI-6 does it with two push buttons. I cut the speedometer cable behind the right chrome lower cover. I installed two connectors (In/ out) and tested both units. Both units work successfully in the GL1800. After calibration with my GPS, I virtually have no speedometer error. By the way, GPS speed accuracy is not a function of speed. A conservative number is ½ foot/sec, which translates into .73 Mph. So at 75 it is very accurate, at 1 Mph not so accurate. However, given multiple reading with different satellites (good DOP), this error is virtually 0. I permanently installed the SGI-6 since it is easier to set and is weatherproof. The SGI-5 would require another enclosure. 

The SGI-6 has two push buttons, up and down. Each depression results in a .5 mph change. This of course also calibrates the odometer. The odometer is offset by about 1.5% (low) from the speedometer. This is done internally in the speedometer firmware. It's apparent that Honda's original design goal was to have the speedometer read 1.5% 
high and have the odometer correct. Why the speedometer reads so high on production models is anybody's guess. With this implementation it's your choice as to whether you adjust to the speedometer or the odometer; but you can not change the relationship between the odometer and the speedometer.

• Note that the speedometer correction device cannot be used on ABS equipped vehicles. 
  • The speedometer input from the transmission is completely separate from the two ABS wheel speed sensor inputs on the GL1800. My bike is an ABS model that I installed the SGI-6 on.
• But doesn't the ABS computer also use the speedometer input as well as the wheel speed sensors? I am asking the question.
  • Good question. If you study the maintenance manual (section 15-B, wiring diagram on page 22-4 and the ETM section 6, wiring diagrams on pages 6-0 and 6-1) the Vehicle Speed Sensor (Speedometer) does not interface with the ABS system. There is no interface between the Engine Control Module and the ABS computer. The ABS is a very independent system and I'm sure intentionally so. The concern would be to vary the input from the wheel sensors. This could cause the ABS computer to falsely detect a wheel speed difference indicating a possible skid. Dakota Digital warns against modifying ABS speed signals, not against modifying an ABS equipped vehicle.

Sticking Speedometer

Honda authorized my dealer to exchange speedometers due to the sticking problem. Took the mechanic total of 6 hours; first time he took the whole front fairing apart. Excellent job, new speedometer did the Honda Hoot from Harriburg to Knoxville and back without any sticking whatsoever. Does require reset of odometer back to zero though.