You should be able to calculate it. Each tire revolution is 70 inches long (22 times pi). There are 1,267,200 inches in 20 miles so 18,100 tire revolutions. There are 60 minutes in an hour. So 300 revolutions per minute. Then whichever gear you are in ratio which I leave as an exercise for the interested. The rear shimano MF-TZ21 is a 14-28; dunno what the chainring is.
Thanks, I will figure it out when I get mine. Just wondering if anyone has a bike computer and monitors cadence, and has noticed if they were pretty much spun out at the top speed of 20 mph. I'm not too comfortable spinning at much over 85 rpm.
I just counted, the chainring is 42. So your cadence will have to be around 100 (42/14=3 300/3 is 100) to go 20 mph. That correlates to pretty much everyone's experience, you can't pedal fast enough in PAS5 to actually help the bike. You'd have to change the chainring to a 60T in order to get the needed cadence down to a comfortable 60. I don't know if a 60T chainring can be fitted, but you can ask your local bike shop after you get your bike (you'll need more links in your chain, as well). Note that moving the chainring to a 60 increases your lowest gear from 1:1.5 to 1:2.1 which isn't great for climbing hills.
After we've had this exchange I've been working at it. Seems that I can help the motor in PAS5 if there is a headwind or slight incline. Cadence speed seemed near 100 as predicted at 20 mph on the display (I don't have a cadence computer on the bike, but I was pedaling very fast).