Speeds vs Watts/Power.. Accurate?

Just have a question about watts and speed. I use a “dumb” trainer; Tacx Blue Twist with a Wahoo Blue SC sensor. Are the speeds relatively accurate in Zwift?

I’ve never hit 72km biking before. I was biking at a 145+ watt; close to 155 I think. And I was going down a -8 grade hill. Does it make sense to have hit this speed? I have no real life to compare it to, as there aren’t many hills around my bike trails.

Sorry I’m a noob and still learning Zwift.

This might be a good read for you: http://zwiftblog.com/zwift-speeds/

And these could help: http://zwiftblog.com/?s=Speed

 

Thanks Paul. I read that first article today on my way into work, but I will check out the others.

I’m assuming, I should just gauge my effort based on my watts/power? It makes the most sense on my trainer.

Speed is a byproduct so don’t use that to judge your effort. Like you said it is best to use watts to judge your effort.