On rough ground you would be happy to go half of the max speed - being thrown around in a tank is most unpleasant, and sometimes right rangerous.
I tried to close the commanders hatch when we drove over uneven terrain in summertime, of course we bounced and the hatch almost gilliotined my fingers, and at the same time my gunner broke his nose on something, we probably did about 30kmh, both of us ended up in the hospital.
Max speed of my tank type was close to 70km/h (on paved road)
Deep snow was wonderful to drive in, until you found a snowcovered ditch, where you had to shovel yourself out - or another tank tow you out of it.