Probably the first thing to consider is the trucks have their speed calibrated periodically to ensure the accuracy of their tachographs (in the UK at least) so a truck doing 90kmph may show as 100kmph+ in a passenger car, I know my Volvo is 7% out, and my Seat is closer to 10% out.

That said, depending on the truck, there's fuses you can pull, ECU remaps and even for the older trucks with the magnetic sensor in the gearbox, the trick is/was to stick a magnet on the sensor (with a bit of string, so you can pull it off remotely if you get pulled over). All of these methods are becoming less feasible, as things like the aggregate wheel speed sensors used for ABS get used, you can't just fool one thing now.

As for the weight limit problem, that's a whole other rabbit hole!