For example I have found "elsewhere" (
www.wwiitanks.co.uk) M3 Halftrack (or more precisely M3 75mm GMC) cross-country speed stated at 35mph, which is ca. 56kph. Unfortunately the source doesn't state cross-country speeds for 250 and 251 but given stated roadspeeds (37mph for 250, 33 for 251)... So the sources (and even the very definition of cross-country speed) may and do vary.