Car crash, Seatbetls, impulse

Also ive been trying to get my head around an equation for seatbelts. Hyperphysics has an auto calculator and ive been trying to do it algebraically and still cannot get the same answer.

Here is what i have done...

In relation to a car travelling at 50mph with a driver which weighs 75kg and stops with in 1 foot.

Car Crash Example

m = 75 kg, initially travelling at 50kmph = 13.88888889m/s d= 30 cm

Distance time interval =

dt = 0.3 m / 13.88888889 m/s = 0.022 s

During this 220th’s of a second it is decelerated from 13.88888889 m/s to zero, so

a = dv/dt = 13.88888889 m/s / 0.022 s = 631 m/s^2

So the impact force is

F = m x a = 75 kg x 631 m/s^2 = 47348N ....

But it is completely different answer