[Uncle Edgar]

Hi Vimes !
I'm going to play mindreader here, and suggest that the higher top speed was so trucks would have longer range at normal cruising speeds - so they can properly fulfil their in-game role as long distance cargo carrier. But I agree that one unintended consequence of this is that they get very high top speeds when not hauling a load.

Suggestion I:
Top speed is 100, always. Increase the range at all speed by the fraction of maximum load you are carrying. So if you're only at half load, you get twice the range.

Suggestion II:
Calculate top speed as usual, but the truck cannot exceed 120mph without damaging the engine. The higher "top speed" from having more power doesn't let you go faster, but does allow you to cruise further.

Suggestion II:
Use the car top speed equations, tweaked to reduce the possible top speeds. Use R to represent the power/weight ratio. When fully loaded, R is 1. When unloaded R is about 3-4, call this M.
The original top speed equation is T = 200R/(R+1). turn the two constants into variables so you can tweak the equation: T = AR/(R+N).

You can now pick what you want top speed to be when fully loaded (T1) and unloaded (T2), call the ratio T2/T1 = U, then work backwards to get the top speed equation:

T2/T1 = U = (AM/(M+N))/(A/(N+1)) = (MN+M)/(M+N)
Rearrange to get:
N = M(U-1)/(M-U)
A = T1*(N+1)

Example, I want the new top speed to be 80 when fully loaded, and limit it to 160 when fully unloaded and M = 4. The ratio T2/T1 = 2. The two constants become:
N = 4(1)/(4-2) = 2.
A = 80*(3) = 240.
Top speed equation becomes: T = 240R/(R+2)

If you do reduce top speed, I'd also recommend increasing the default range.