Ah… NordicTrack. I bought an Incline Trainer last year (as did Scott J.) and both of ours under-report the actual speed. And the inaccuracy increases as the incline increases. So I would take speeds on NordicTrack machines with a grain of salt and go by heart rate (or calibrate the treadmill).
I know it’s a pain, but I would do another test to be sure. However, it does sound like you can go at higher heart rates than you thought previously.