Perhaps I am mistaken here or I haven't done my home work but I looked it up here
https://wiki.factorio.com/Prototype/Mov ... ment_bonus
and from what I understand (and this conforms to my experience with modding movement bonus equipment) the movement bonus of an equipment applies to top speed and turnrate.
That means that a vehicle will get.. for example .. +25% speed -and- turn rate from a movement bonus equipment.
My idea/suggestion is: Separate movement bonus from equipment into Speed and turn rate, but not both at the same time.
Why am I suggesting this? - vehicles are bit iffy. Not only do they tend to be fragile (yes, car, that means you
) but they can also be very hard to control if you use movement bonus equipment on them. Not that they are easy to drive to begin with 
Why do I think that my suggestion is helpful?
Having speed/turn rate bonus separated allows for a much finer calibration of movement bonus equipment. For example you could increase a vehicle speed by say ... 25% but only increase its turnrate by 5% or 10%. I feel this would allow for a much more refined vehicle control in mods and perhaps even in the standard game. I mean who hasn't tried driving a car around and made some very abrupt close-up encounters with trees for the most part but also rocks. No offense to either trees or rocks here but I just like'em from a distance
Can it be done?
That one's for you, smart people
I have no clue ...