[Imp]

While I agree to some extent some vehicles are to fast off road in my view. Worst offenders are perhaps small fast WWII utility vehicles like the jeep which are faster than Land Rovers & Humvees off road despite the later having superior off road capability.
Now your looking at specific vehicles capabilities & tailoring the speed for that & there are 1000's of vehicles.

These are the only ones that strike me as maybe being worth adjusting slightly long ago I checked vehicle speeds & on the whole they were pretty good, certainly far better than a similar game. Its a simple calculation & some vehicles will be off due to good suspension drive trains or whatever, its not perfect but its not bad considering.

The problem with your calculations is you are not looking at what hex tiles represent & you probably need to generate maps with more fields rough hexes.
Clear - is just that grass dirt with the odd bit of vegetation & maybe a few minor undulations.
Fields - can also represent rougher terrain more vegetation, hedges fences etc hence it causes hinderance to sight & movement.
Rough can be anything from rocks -graveyard - small gully etc etc. Something that needs to be driven through carefully.

So the problem is really you want maps with less clear hexes & more features including other terrain types that adversely effect wheeled vehicles more than tracked.
You can adjust with the map generator or choose batlocs to suite what you want.

Using your example of a Motorbike round where I live doing more than 45kph off road is not a problem in most places unless the grounds wet. Need to slow for some things so that would be a rough hex, field or stream.
Taking into account the scale of a hex however I can drive around a lot of this difficult terrain & still be in the hex so the majority of the open terrain around me would be clear.
One area has some small not nice hills undulate badly, rolling hills would be clear these should be fields with some rough in steep areas.
Going up the mountain near me is a different matter entirely it should be mainly rough hexes then become impassable terrain.