Vehicle max speed formula

[Adil]

Does anybody know which formula is used to define the vehicle maximum speed?
There's no explicit definition of top velocity in the prototype, so I've figured it's some sort of balance between friction and engine power, but attempts to approximate friction with linear and square functions did not succeed.

I've tried to describe the following vehicles

Vehicles

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tank={effectivity = 0.6, consumption = "600kW", friction = 0.002, weight = 20000, measured_speed=0.17}
car={ effectivity = 0.5,consumption = "150kW", friction = 2e-3, weight = 700, measured_speed=0.33}
raven={effectivity = 0.3,consumption = "200kW",friction = 1e-3,weight = 500,  measured_speed=0.66}

with these formulae:

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max_speed= math.sqrt( effectivity*consumption/ weight / friction / const)

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max_speed= effectivity*consumption / p.weight  / p.friction/const

which stand for viscous friction and linear one (assuming that friction value in prototype stands for friction force coefficient: F=velocity^some_power * friction).
I used car as a reference point to set a constant multiplier.
However, first one undershoots the speed of raven vehicle and the second one is wrong for a tank.

edit: the cube formulae doesn't seem to work either.