Originally Posted by Abraxas
Walter, just to confirm, your speedo is in km/h, isn't it?
Yes ... km/h
I had manually (incorrectly) set the speedo calibration, and at a stable speed it actually underreads by about 2.5% (stock speedos overread by about 5% at 100 km/h) ... so on mine if the speedo is saying 100, the GPS is normally saying 102 or 103. Its often a bit hard to read the GPS, as the screen protector kinda makes it hard to photograph the screen well, but in the shot here you can see the GPS ... which is reading 114 km/h while the speedo is reading 112.
On a stock speedo, if speed was stable and the GPS was reading 114, then the Speedo will read about 120.
Terry had a stock digital speedo in mph, but ran his GPS in metric (km/h), and while Terry and I both saw top GPS speeds of 132-133 km/h on the double track trails his speedo got to 90 mph at the same moments, which is 144 km/h ... about an 8% overread. The faster you go, the more OEM speedos are inaccurate. My experience is that most OEM speedos are out by 5-6% at 100 km/h and about 10% at 150 km/h - which is similar to what we were seeing from Terry's OEM speedo as well. The aftermarket speedo I was using doesnt use a built in inaccuracy curve. Its out by a fixed percentage all the time, in my case -2.5%. If I had adjusted it correctly, it would have matched the GPS at every stable speed.
[If you are curious, UNECE rules for speedo accuracy on motorcycles specify they can NEVER underread. So thats why they are made with built in overread at the factory - and technically my bike was illegally modified as it had an underreading speedo!. Maximum overread for production bikes is 10% plus 8 km/h ... i.e. a bike cruising with a GPS reading of 120 km/h can have a OEM speedo reading as high as 140 km/h.]
Also note, if accelerating rapidly, then the GPS obviously lags, as it is a lagging indicator (indicating average speed over the last 20-30 metres), whereas the speedo is far more instant, updating speed each front wheel revolution (every 2.2 metres).
The number under the speedo is the tacho ... which usually worked ok ... so in the pic you highlighted its 104 km/h at 5300 rpm, and in the pic below, its 111 km/h at 5600 rpm