Thanks Bluecat. My reference is a garmin GPS and both the realtime speed and the average after a trip is around 10% off. I consider the garmin GPS (especially the average after a long trip) fairly accurate.
However this is the problem: When the Omni show 45 km/h I'm only doing around 41 km/h on the Garmin. I assume this mean that the Omni shuts off assist (and I can tell by the assistance bar on the omni going almost all the way down) before I actually do 45 km/h which means that if it was correctly calibrated I would be able to actually get assistance to 45 km/h which I don't get now.
I contacted Stromer about the issue, and they said I could contact my LBS about it - but LBS is not responding (and moved the shop from retail to web).
Any input still greatly appreciated. Thanks!