Given those figures I get the same result: one wheel at a time would lift at speeds above 103 mph. The ride would surely then become very rough indeed, whatever it was like below that speed. However we don't know (or do we?) whether the proportion of reciprocating weight balanced to give those figures was the same as on 2903 at the time of the alleged event. I see no reason to disbelieve Collett's account, both that they went very fast and that they had only a very rough idea quite how fast. If the balancing was as above then the speed can hardly have been more than 103. But maybe the balancing then was different, in which case the speed might have been higher, though anywhere near 120 mph seems extremely implausible.