Speedometer reading slow

I know this is late, but I had this problem with a new Cateye in 2016, Same size wheels and tyres on different bikes, different reading. But the bike I have now An Malvern Star Oppy s1 it is reading pretty close to correct. Test with wife driving car. So I don't know what the problem was, but LBS stated that speedo's are not all that accurate. GPS is a better idea instead.

peters3103 said:

Hi all, newbie here so be gentle,
I have s Sigma BC506 cycle computer which I have removed from my old road bike and fitted to my new Giant Roam 2. It seems it’s reading too slow. I have entered the correct wheel size
(38-622 for 700x38) and the pickup is mounted correctly with a 2-3mm space. When riding at a reasonable pace it’s showing about 11km/h and when I pedal as fast as I can on the flat it reads 19km/h. This seems lower than on my previous road bike. Any tips?
Cheers, Pete

This may be stating the obvious, but a 622-38 tire is a rather robust tire size ...

And so, if your previous bike had 622-28 tires, for example, then you will almost necessarily be going slower for the same pedaling effort than before.

And, THAT may be why your speedometer is reading a slower speed.