Zwift running - Inaccurate speed

I tested this morning Zwift for runner. It’s seems promising but the most important feature, running speed, is inacurate.
I’m using a Garming FootPod bought in 2012 (model:01397) and the speed of my treadmill (and my speed feeling) is constantly faster than what the pod report.
For example, 8kph is reported as 5kph, 10kph as 6kph and 15kph as 10kph. This is quite annoying.
Would anyone have noticed the same thing?

You will probably get more assistance on the Zwift Runners group on Facebook since running on Zwift is in pre-beta and not supported.  

I don’t think the problem is for Zwift. I have the same problem with my Polar Pod and my watch. When I am on an indoor track, my speed look accurate but when I’m on a treadmill… the speed on my watch is about 40 sec/km slower.