[Imp]

I will just say I think your perception of driving off road is wrong
been a while since I 4X4ed but you assume difficult terrain is more frequent than it is in a lot of places in the world. Some places really are not vehicle territory, not talking a few hexes more like you could have crossed the map on foot just as fast.

But keep in mind hexes are 50m across there is more often than not a fairly fast route through it or as in the game drive through another hex or 2 to go round the problem.

A 4x4 with off road tyres that you don't mind scratching the paintwork on collecting bushes & if military driving through fences etc can go places that would surprise you.
Most of the time is NOT spent in low gear if you have a low/high box low is only used for heavy mud or really steep hills normally. It's very useful coming down steep hills on loose terrain which is dangerous & has to be done dead slow as the brakes will just lock.
Same loose sand or whatever can easily be done on the level at around 40mph you will slide & have the odd scare on a particularly loose or fine grain bit but as said so long as you don't mind scratching the paint its fine.

What restricts your speed the most is passenger comfort not risk in most places plus repair bills you will break suspension & drivetrain components over time.

Oldest 4x4 I have driven was a 1980,s Land Rover top speed around 70mph, never got it above 60mph off road normally around 30-40mph because as I said its uncomfortable. Rubbish on the road but I thought it very capable off road you could go virtually anywhere.

Then I got to drive one of the work dump trucks its wheels were about 3 times the size of the Land Rovers, slightly bigger than those fitted to modern military vehicles.
Boy did I get a surprise smooth ride bigger wheels do not fall into the ruts I can go 20mph faster in greater comfort virtually everywhere. I can also make my own path to a large degree small shrubs & bushes pass underneath unnoticed you don't even feel them.
The odd thing was it had better traction & hardly moved about at all steep inclines if loose or very muddy were the only things that needed care & maybe a push from a Dozer.

This I would assume is more akin to a military vehicle it weighed something like 25 tons loaded had large wheels

What I am trying to explain to you is if you go down a road with a bad surface & potholes in your car the ride would be better in that truck doing 50mph over one foot high bumps & ruts.
Your 10kmh is the speed you would drive over an obstacle the size of your car so you don't tip over.