Is immobilization excessive?

[Dog829]

Here I am referring to German and Soviet WWII era vehicles. Primarily, I have been playing Red Victory both vs the AI and pbem. The high number of of non combat, mechanical vehicle failures I have experienced playing either side has led me to wonder whether it's excessive. I haven't recorded exact numbers but I estimate 15 -25% is about right with the 20-25% number happening more often.
In a game I started earlier on turn 9 of 30 and playing as the axis in August 1943 I have lost three tracked vehicles or 10% already in normal conditions while moving on secondary roads.
What's normal for WWII vehicles?

From the users manual on Immobilization:
For a given movement from one hex to another with a movement cost of C and the Move Immobile Factor parameter data value F, the probability of the movement causing the Vehicle to become Immobile is C * F.

Where can I find the data which constitute F? It seems it can't just be the vehicle reliability data value found in the weapon data dialog since it happens so often to vehicles with reliability ratings of "A" that something else must be factored in besides random chance. Terrain? terrain conditions? Time of year? Year? Status?

Thanks