I could be slightly out of touch of the 'cutting edge' with this but:
"modern tractors can do upwards of 40-50mph"
Most can't though
Off the top of head exceptions would be:
JCB fastrac, factory limited and 40mph legal. I've heard they can be chipped to go 50mph+ but there might be possible warranty and perhaps insurance issues
Mercedes Unimog, not really a tractor, more an offroad lorry that will take agricultural implements. Not sure but geussing going to be 50mph+. Price tag goes toward explaining why there aren't many about
There is/was a Mercedes Sprayer than would do 50mph+ or at least one that overtook me on the A1 must have been doing that speed. Nnot impossible it could have been chipped?
Some Fendts I think can physically do 60kpmh (~38mph). I don't know how they fit in around UK 40mph regs though
There could be others?
"always do exactly 40kph"
They'll be ones limited to 40kmph, either revs in top gear for economy or don't have the gears to go faster
40kmph is normally 'base model' speed, with 50kmph an optional extra for many. Some 50kmph as standard or more
"big John Deere 6920"
big? big?! pah ..
"60kph 'boxes but there's certain criteria for them (independent wheel suspension, air brakes and possibly abs"
Along with the independant wheel suspension I think having the steering wheel mechanically linked* to the steering wheels is a must too? JCB fastracs** and unimogs also have rearward opening doors (like most cars), though not sure if this is a requirement
*most tractors only have the power steering element of the typical steering setup on a car, ie no steering rack or other mechanical linking between steering wheel and wheels, which I think dictates the UK speed limit for most tractors displayed in most of the cabs. I should think (almost?) all the 40kmph and 50kmph tractors out there fall into the UK legal road speed limit
**At least the 'original' shape/layout fastrac (and mismatched wheel size one used on top gear). The 'conventional' layout ones have forward opening doors, though not sure if they are 40mph capable?