I am 265lbs. and 6'5". It was pretty windy and my neighborhood does not have a lot of long level roads. These are not "exact" and this is made a little harder because there is a decent damping/time delay on the speed-o of the bike.
General rule of thumb: subtract 10 off the speed-o for any indicated speed 35mph or above.
40mph indicated is around 30.5mph GPS
45mph indicated is around 36.2mph GPS
The best I could do at a top speed test in my neighborhood resulted in a 37mph GPS top speed. But I was going into the wind and I am not sure of the flatness of that part of the road.
Hopefully I can find a good calm day when I have more time and take it out for a longer ride on some straight and level roads.
For me on my Z it seems like a reasonable estimation of top speed is 36-38mph.
I'll post more as I get better information.