The speed limit of a car will depend on the power of the car (unless it has specific speed limiters). You need the power for the accelaration in case of an emergency. Also, the manufacturers have to take into account the weight. If your car only had the power to have a maximum speed limit of 70mph normally, you will piss off a lot of drivers behind if you filled it full of kids, with a roof rack of luggage, tow a caravan while driving up hill on a single carriageway.