[Imp]

Cross country speeds are never going to be accurate they are based on the unit class with no modifications for specific units.
Book cross country speeds are also suspect, some may be wrong, Cromwell 16 hexes really.
Its probably much like armour penetration what do the testers class as open ground.
Playing field, steep hill, hard or soft going, fields or rough maybe a small obstacle to climb.
A Tiger drivuing over a rough fields etc wouuld be closer to 3 than 6 hexes.

From Chris my serious offroad budy weight matters makes getting the rest right harder if to heavy. Ground pressure is far more important. Horsepower is not very important torgue is especially if the vehicles heavy.
The most important thing though is probably the drive train & suspension & this is the where you get the big variations.
A Land Rover can still outperform many vehicles once the going gets tough despite being underpowered.
He runs a heavily modified Nissan something wide tyres specialised drivetrain & shocks, motor tuned down to about 320hp from over 500 to produce serious flat torque curve because thats what you need offroad

In game this would just be represented as any 4x4 at the level of this game you cannot take into account the individual aspects of the vehicle like how good the suspension is off road.