Speedometer versus GPS Indicated Speed

[Merritt64]

Does anyone else have a discrepancy between their indicated speedometer versus their Garmin indicated speed?  I have noticed a discrepancy of about 8 km/hr at 120 km/hr.  My speedo will show 128 km/hr while my Garmin shows 120 km/hr.  The original tires are on the bike and have about 10k km's on them.  At lower speeds, the discrepancy becomes much less, therefore, it must not be linear.  IIRC, I will have a discrepancy of about 4 km/hr at 80 km/hr with the speedo reading higher again.  I don't know which one to trust.

[Coconut]

This has been discussed in depth previously ....

Speedo Accuracy - Runs to 10 pages

Summary - GPS indicated Speed is likely to be more accurate than the Trophy Speedometer
when riding along a flat / level road ( i.e. not going up or downhill ).

Cheers 

[Merritt64]

I apologize.  I didn't search the subject prior to posting.  Thanks for your help Coconut.

[kirkkw]

In addition to the one linked by Coconut here is another one:
https://www.triumph-trophy.com/index.php/topic,274.88.html

I haven't read them both but have read a few pages in both of them.  It seems there is no remedy but to ignore it.

My CTX1300 has a similar problem that got solved when I dark-sided the rear tire.  Now it is within 1 MPH of the GPS.

[digital]

It is one thing that I have never given any importance to. What's more, it is better that the motorcycle's speedometer always go a little more ahead than the real speed.

I carry the TomTom 550 and it gives me 7 to 8 kilometers below the speed of the bike.

[dietDrThunder]

Ya, this is the case on pretty much every motorcycle ever. When I went to the 200 rear tire my speedo got closer, but I just use my GPS.

[T800XC]

Originally Posted by digital

...it is better that the motorcycle's speedometer always go a little more ahead than the real speed...

I think there's a very good reason for that...

I don't know the rules for other parts of the world, but in the EU I think the following EU Directive (2000/7/EC) for 2- & 3-wheeled vehicle speedo accuracy is still valid:-

The displayed speed must never be less than the real speed.
So it shouldn't be possible to get caught speeding if your displayed speed doesn't exceed the road limit.

The displayed speed must not be more than 10% greater than the real speed, plus 4km/h.
This would give a maximum displayed speed of 134km/h for a real speed of 120km/h, or 79.5mph for a real speed of 70mph.

This applies to bikes/trikes fitted with the correct manufacturer-recommended tyre type and at the 'normal' running pressure.

In the UK the unofficial speed prosecution rule of 10% over the limit, +2mph, still falls within above EU speedo regulations for all posted speed limits from 20~70mph.

Sat-nav indicated speeds should be far more accurate but, as others have said will depend on, amongst other things, the quality of the GPS reception and whether you're travelling on a straight & level road.

[digital]

In the best place that I always see that difference every day when I return home, it is on a section radar of about 8 or 10 kilometers that that section radar does. I set the cruise control to 120 km / h, the speedometer of the motorcycle is constant at + - 1 km and the GPS usually stays at 114 km / h.