Unfortunately there is no way to normalize speed readings in TrainerRoad at this time. The difficulty here is that, when you really think about it, any mileage is “false mileage.” You didn’t move anywhere during your workout 
This creates a challenge because in order to estimate mileage, we would need a physics engine to process your power, weight, and estimated aerodynamic drag to get a virtual speed/distance reading. As a company, we feel that power is the best way to quantify your workouts since you have metrics like TSS, IF, and FTP to monitor your training and progress. For this reason, we do not have immediate plans to develop the tools neccessary to more closely estimate your speed/distance.
As Chad mentioned, you are more than welcome to alter your own gearing in order to achieve the speed/distance you want to see, but as you said, you can’t make your riding mates follow the same rules.
Sorry I don’t have a better solution for you at the moment.