That one calculates an even higher number than the one I linked to, though not much higher.

I feel pretty good about the mph because of my experience on walks that were GPS tracked. 4.7 is about the fastest I can walk and the reason I can't maintain that very long on "regular" walks is irregularities in roads/sidewalks, turns and inadvertent drops in speed; without the readout, it's easy to taper off a little without noticing.

I do wonder about fitness level also. It will be interesting to see if my HR drops if I keep doing the treadmill with similar settings. I am thinking of dropping speed to 4.5 and increasing grade to 10%. I think pushing the edge of the envelope on walking speed is asking for trouble with joints.