You were inside on a stationary bike.. you had no wind resistance. Wind resistance is huge. If I have a nice tailwind I can run at 20 MPH like it's nothing.. if I have a headwind 7 or 8 MPH can be torture. With no wind I'm usually around 14 or 15.
BTW, I'd like to second the idea the a car likely won't notice the difference between you riding 10 or 20 MPH unless they're following you for an extended time.