Just once, my Dad achieved an indicated 100mph (on the chronometric) with his plunger Flash with double adult sidecar, Mum on the back & 4 kids in the chair.
It was on a long, straight and very steep downhill run in North Wales, so the extra kinetic energy will have helped.
I have no idea what preparation he had done and what remedial work was needed afterwards, but I can still hear the whoops of both parents.
We stopped for a picnic afterwards - primus to make the tea to drink out of jam jars as usual & a bottle of Sterry with the crown cork carefully pressed in place for later.