In ERG mode, the speed(if I understand it right) is essentially just the speed/RPM of the flywheel on my Kickr Core, and doesn’t change with resistance. Workout is the same wattage whether I do 39x28 or 54x13, but the higher the gear, the faster the flywheel spins, and the “farther” and “faster” I go. I typically do mine in 39x19 or so, and the speed works out to about 28kph, or thereabouts.
Speed and distance are of course worthless metrics on a trainer. However, I do have a goal of 14,500 km I’d like to reach for the year, a carrot on a stick if you wheel, so I do like to get a reasonable distance value from a trainer workout, so that I give myself “credit” in a mileage sense for the time I put in.