Classic Trainer Speed Difference

Just curious how different the speed difference should be with a classic trainer from what my computer says to Zwift. I’ll be riding at 15 mph on my computer but it’ll be 3 mph on Zwift.

In the sensor setup my speed matches up there, just not the actual ride. From reading it sounds like there’s some conversion with speed and watts but I guess I’m still a little confused. Any help would be appreciated.

I selected classic trainer, tire size 700x23, and I use my laptop with a usb ANT doogle under the bike. It had the correct speed for a second on one test but I couldn’t get back to it.

Please read this:

And ignore the speed that your bike computer is displaying.