Holy crap it works! Wow, thanks, I thought it would be trickier than that so I didn't get the calculator out.

Here are my workings and they are astonishly accurate:

Size 1 = 434px
Size 2 = 232px
Distance 1 = 80.3m

Therefore (434* 80.3m) / 232 = 150.2m (the far distance)
Subtract 80.3m then divide by 4.06 seconds = 17.2m/s = 38.52mph

Given rough measurements and the fact that the distance could be up to four frames out, that isn't bad!

Gareth