Speedo Inaccuracies

My Speedo is about 5-8mph slow across the board. Is ths a common thing on the Silverwing?

The speedo is driven by the back wheel sensor.
Are you using the correct tyre profile of 150x70x13, 70 being the profile/height of tyre.


Be safe, be seen!

Yes, the speedo does have a  5-8 mph error with stock tire size. However the odor is spot on.

For about 40,000 miles I ran a Sumitomo 165/70-R13 darkside tire. The speedo was spot on but the odo was then off by that 5-8 mile error.

On all my Silverwing's I ran the same size tire and when 60 MPH indicated My GPS shows 55 MPH. This has been consistent on 3 bikes.

Murf wrote:

On all my Silverwing's I ran the same size tire and when 60 MPH indicated My GPS shows 55 MPH.  This has been consistent on 3 bikes.

This is my case, exactly, so I use a GPS speedometer regularly (actually a bike computer)

I think every vehicle I have owned has had a speedo error.
TBH I've never worried about it, it give me a little leeway with speed cameras etc.

I once read that with a "darkside" tire installed that the speedometer reading would more acutely reflect the "true" speed... (just another two cents worth)...

Looking a bit further into this is seem it is due to legislation.

Euro and UK standard dictate that a speedometer must not under-read, so manufacturers set them to over-read to a little.
US law is different in that a percentage inaccuracy either way is allowed.

So there you have it.
All those booked for speeding in the US are innocent, it's all down to instrumentation inaccuracies, meanwhile in the UK/Euro it's because we are evil minded, irresponsible hoodlums and should thrashed to within an inch* of our lives.
* Or should that be 25.4mm?

Further to what Steve has written; I understand the margin in the UK built-in is 10%, a 'fact' born out when comparing speedo with GPS.
I suspect that this is why so many motorists dawdle at 10% under the speed-limit.

I've noticed that since I triked out my SWing the speedo is more accurate. Almost right on. The two back wheels are 14" tires so that might have something to do with it.