Zwift shows 6mph when it should be 18mph

Just logged in to my first ride and was pretty disappointed. My bike computer showed 18mph but Zwift was only registering 6mph. My cadence was spot on. 

Using a basic setup with a Wahoo Blue SC sensor

Hi Adam, what trainer are you on? 

The speed your wheel is spinning on your trainer has no direct correlation as to the speed you’ll go in Zwift.  For example, if your avatar is climbing a hill in Zwift and you’re on rollers you might be spinning at 50mph in your top gear while you’re avatar climbs the steep hill at 4mph.  We take the watts you generate and put it into the physics simulation of the virtual world.  Raw speed is meaningless there.