Zwift speed wrong

First time using Zwift, speed way off the mark. Showed up as 2-4mph even though pedalling quite hard. Cadence was accurate. Stayed similar for about 5 minutes then went through the roof and I topped 40mph even though my gearing and cadence hadn’t changed. Got fed up with this after a while and checked dongle and other equipment was working correctly by switching to Trainer Road (which I have been using prior to my experiment with Zwift) and it all showed sensible figures and matched what was showing on my Garmin 1000. Only explanation seems to be software issue. Anyone any advice?

Zwift speed will hardly ever match the speed which is show on your Garmin. The Garmin does not know about the virtual elevation changes within Zwift.

Zwift speed is calculated by taking your watts, weight entered, in game bike used, in game drafting and virtual elevation changes.

I’m guessing when your speed was low you were doing a virtual climb within Zwift and once you came over the top and descended your speed increased.

I agree with everything Paul said.
I also think for a given power output, on a flat road in Zwift, my speed is about 2-3 mph faster than I would expect on a flat road in real life in the specific gear and exertion.
I don’t have power measurements IRL.
Namely, cruising around Zwift at 22 mph seems to be alot easier than the same speed in real life.
I use a wheel on dumb trainer and there is a lot of inaccuracies associated with this but that’s not what I’m talking about.
The trainer and zwift power issues will cause your power (watts) to be falsely elevated.
I’m talking about your in game speed based upon a given power output.
When my power is about 160-170, my speed is freq 20 mph but in real life, I feel I would be working harder.

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