The speed on your trainer, as it relates to the road is completely irrelevant.

The only reason you would even care about speed on the trainer is if you're using the KK power curve to estimate your power.

So don't worry about how it correlates to the road.

You should be measuring your efforts on the trainer by time and intensity. And you measure the intensity by perceived effort, HRM or a power meter, in order of preference.

Again the only way you would use speed on the trainer is to estimate power against a known power curve. And what that curve says will likley have little relevance to your power at a given speed on the road.